Partly revert r295168 and define PTE_DEVICE in pmap-v6.h header again.
It turned out that devmap.c is not only file in which PTE_DEVICE
is used and simultaneously, built for both armv4 and armv6 platforms.

When I tried to build all arm kernels before r295168 commit, it was
hid by some other local changes in my tree. I hope that this is just
temporary workaround before VM_MEMATTR_DEVICE could be used instead of
PTE_DEVICE outside of pmap code for __ARM_ARCH < 6.