of/fdt: Clean up early_init_dt_reserve_memory_arch()

As of commit 18250b43 ("of: fdt: Remove
early_init_dt_reserve_memory_arch() override capability") this is
no longer an arch hook, so rename it to remove the confusing _arch
suffix. Also remove some unnecessary indirection from all but one of
the callers by calling memblock_reserve() directly instead.

Signed-off-by: Peter Collingbourne <pcc@google.com>
Link: https://linux-review.googlesource.com/id/I3362bdd92ae6e47e8f5bac01aa228d32f9d01aad


Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220723015331.1607029-1-pcc@google.com