nommu: Provide stubbed alloc/free_vm_area() implementation.

Now that these have been introduced in to the vmalloc API, sync up the
nommu side of things. At present we don't deal with VMAs as such, so for
the time being these will simply BUG() out. In the future it should be
possible to support this interface by layering on top of the vm_regions.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>