Mapping non-cachable memory as cachable and subsequently changing the mapping

Mapping non-cachable memory as cachable and subsequently changing the mapping
to non-cachable is retarded.  Fix this by introducing PMAP_NOCACHE and
PMAP_DEVICE flags that can be or'ed into the physical address passed to
pmap_kenter(9), like we have on many of our other architectures.  This way we
can also properly distinguish between device memory and normal (non-cachable)
memory.

ok visa@