vm: add no-mmu vm_iomap_memory() stub

vm: add no-mmu vm_iomap_memory() stub

commit 3c0b9de6d37a481673e81001c57ca0e410c72346 upstream.

I think we could just move the full vm_iomap_memory() function into
util.h or similar, but I didn't get any reply from anybody actually
using nommu even to this trivial patch, so I'm not going to touch it any
more than required.

Here's the fairly minimal stub to make the nommu case at least
potentially work.  It doesn't seem like anybody cares, though.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>