xtensa: remove arch specific early DT functions

Now that the DT core code handles bootmem arches, we can remove the xtensa
specific early_init_dt_alloc_memory_arch function. The common
early_init_dt_add_memory_arch can be used too now that xtensa switched to
memblock.

Cc: Chris Zankel <chris@zankel.net>
Cc: linux-xtensa@linux-xtensa.org
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>