Fix the armv[67] memory attributes for uncached memory.  Previously it was
mapped as strongly-ordered which meant that unaligned accesses would fault.

armv7_generic_bs_map now maps pages with PMAP_DEV which is treated as SO

bus_dma continues to use PMAP_NOCACHE as appropriate, but this now get
mapped to the correct memory attribute bits for armv[67]

DEVMAP_ENTRY usees a new flag PTE_DEV.

The workaround for the unaligned access faults is now removed.

XXX Other armv[67] boards bus_space implementations should be checked.
XXX There is scope to reduce the difference to aarch64

(skrll)