of: fdt: Remove early_init_dt_reserve_memory_arch() override capability

Commit e7ae8d17 ("MIPS: replace add_memory_region with
memblock") removed the last architecture-specific override of
early_init_dt_reserve_memory_arch().
Convert the common implementation from a weak global function to a
static function.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/be0140a0183ecfd0a3afa4fe6d2d77ed418102f9.1628671897.git.geert+renesas@glider.be


Signed-off-by: Rob Herring <robh@kernel.org>