nios2: remove arch specific early_init_dt_alloc_memory_arch

Now that the DT core code handles bootmem arches, we can remove the nios2
specific early_init_dt_alloc_memory_arch function.

Cc: Ley Foon Tan <lftan@altera.com>
Cc: nios2-dev@lists.rocketboards.org
Reviewed-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Rob Herring <robh@kernel.org>