rpi5_native_boot.c: carve /reserved-memory out of the Pi 5 memory map

Previously deferred (rpi5_native_boot.c's own header comment flagged this
as needing interval-splitting logic written blind against hardware not
yet in hand) -- revisited by fetching bcm2712-ds.dtsi directly rather than
assuming reserved-memory was empty or absent. It has one static child
(atf@0, ARM Trusted Firmware's own region) and one dynamic child
(linux,cma, size/alloc-ranges only, no fixed reg) -- the dynamic one is
skipped, nothing fixed to carve and no allocator this early to service it
against anyway.

Adds two fdt.c primitives: fdt_find_node_by_name() (reserved-memory has
neither compatible nor device_type per DT spec) and fdt_next_child_node()
-- one exported symbol, not the two-primitive general sibling-walker
originally sketched, collapsed after review since the only real use here
is "iterate one node's direct children."

collect_reserved_ranges() reads each child's own #address-cells/
#size-cells with a fallback to root's only if absent -- confirmed
necessary, not just defensive: reserved-memory's own declared <2>/<1>
genuinely differs from root's <2>/<2>. emit_region_with_carveouts()
clips a sorted reserved-range list against each RAM region, emitting
alternating EfiConventionalMemory gaps and EfiReservedMemoryType
carve-outs (insertion sort, no libc qsort in freestanding).
RPI5_MAX_MEMMAP_ENTRIES is the exact worst-case count, recomputed rather
than estimated -- the rpi5_mailbox.c buffer-size bug is the standing
lesson for this pattern.

Compile-only-verified; nothing in the existing UEFI/QEMU path calls
rpi5_native_boot(), so this cannot be exercised until real hardware.
Verified 3-arch boot to ok>/zuse)ok>.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019YcT3H2PQeyujrzjqS3Var

include/starkernel/fdt.h

@@ -102,6 +102,47 @@ const void* fdt_find_node_by_compatible(const void* fdt, const char* compatible)
  */
 const void* fdt_find_node_by_device_type(const void* fdt, const char* type);
 
+/**
+ * @brief Find the first node whose own name matches @p name.
+ *
+ * Node names follow the DT spec §2.2.1 `name[@unit-address]` convention —
+ * matches if @p name equals the node's name up to (not including) an `@`
+ * suffix, or the whole name if there is none. For a singleton node with
+ * no unit address (`/reserved-memory` being the concrete case this was
+ * added for), this is an exact match. Scans the whole tree in document
+ * order; the first matching node wins if more than one exists.
+ *
+ * @param fdt   Blob, already checked with @c fdt_valid().
+ * @param name  Node name to match, NUL-terminated, no `@` suffix.
+ * @return An opaque handle to the matched node, for use with
+ *         @c fdt_find_prop_in_node() / @c fdt_next_child_node() only —
+ *         or NULL if no node matches.
+ */
+const void* fdt_find_node_by_name(const void* fdt, const char* name);
+
+/**
+ * @brief Iterate the direct children of one node.
+ *
+ * Pass @p prev_child as NULL to get the first child; pass a previous
+ * result back in to get the next one. Stops (returns NULL) once there are
+ * no more children. Skips over each child's own descendants correctly
+ * (so a child with grandchildren doesn't confuse the scan), but does not
+ * itself descend into them — only direct children of @p parent are ever
+ * returned.
+ *
+ * @param fdt         Blob, already checked with @c fdt_valid().
+ * @param parent      Handle from one of the `fdt_find_node_by_*()`
+ *                     functions.
+ * @param prev_child   NULL for the first child, or a handle previously
+ *                      returned by this function for @p parent to
+ *                      continue from.
+ * @return Handle to the next direct child, for use with
+ *         @c fdt_find_prop_in_node() / @c fdt_next_child_node() only —
+ *         or NULL once @p parent's children are exhausted.
+ */
+const void* fdt_next_child_node(const void* fdt, const void* parent,
+                                 const void* prev_child);
+
 /**
  * @brief Find a property by name, scoped to one node.
  *