nios2: use generic early_init_dt_add_memory_arch

nios2: use generic early_init_dt_add_memory_arch

All we have to do is to enable memblock, the generic FDT code will take
care of the rest.

Link: http://lkml.kernel.org/r/1533326330-31677-4-git-send-email-rppt@linux.vnet.ibm.com
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Acked-by: Ley Foon Tan <ley.foon.tan@intel.com>
Cc: Guan Xuetao <gxt@pku.edu.cn>
Cc: Richard Kuo <rkuo@codeaurora.org>
Cc: Richard Weinberger <richard@nod.at>
Cc: Rob Herring <robh@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>