drivers/firmware/memmap.c: modify memblock_alloc to memblock_alloc_nopanic

memblock_alloc() never returns NULL because panic never returns.

Link: http://lkml.kernel.org/r/1545640882-42009-1-git-send-email-huang.zijiang@zte.com.cn
Signed-off-by: huang.zijiang <huang.zijiang@zte.com.cn>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Yi Wang <wang.yi59@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>