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

capsules/BLOCK_MAP.md

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