mips: remove arch specific early_init_dt_alloc_memory_arch

Now that the DT core code handles bootmem arches, we can remove the MIPS
specific early_init_dt_alloc_memory_arch function.

Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Signed-off-by: Rob Herring <robh@kernel.org>