of/fdt: use memblock_virt_alloc for early alloc

memblock_virt_alloc() works for both memblock and bootmem, so use it and
make early_init_dt_alloc_memory_arch a static function. The arches using
bootmem define early_init_dt_alloc_memory_arch as either:

__alloc_bootmem(size, align, __pa(MAX_DMA_ADDRESS))

or:

alloc_bootmem_align(size, align)

Both of these evaluate to the same thing as does memblock_virt_alloc for
bootmem. So we can disable the arch specific functions by making
early_init_dt_alloc_memory_arch static and they can be removed in
subsequent commits.

Cc: Frank Rowand <frowand.list@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>