NOMMU: Rename ARM's struct vm_region

Rename ARM's struct vm_region so that I can introduce my own global version
for NOMMU.  It's feasible that the ARM version may wish to use my global one
instead.

The NOMMU vm_region struct defines areas of the physical memory map that are
under mmap.  This may include chunks of RAM or regions of memory mapped
devices, such as flash.  It is also used to retain copies of file content so
that shareable private memory mappings of files can be made.  As such, it may
be compatible with what is described in the banner comment for ARM's vm_region
struct.

Signed-off-by: David Howells <dhowells@redhat.com>