ARC: mm: switch to asm-generic/pgalloc.h

With previous patch ARC pgalloc functions are same as generic, hence
switch to that.

Suggested-by: Mike Rapoport <rppt@linux.ibm.com>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Vineet Gupta <vgupta@kernel.org>