spelling in comments fixup

Submitted by:		Joerg Sonnenberger <joerg@britannica.bec.de>
Begin reducing code duplication in arm pmap.c and pmap-v6.c by factoring
out common code related to mapping device memory into a new devmap.c file.

Remove the growing duplication of code that used pmap_devmap_find_pa() and
then did some math with the returned results to generate a virtual address,
and likewise in reverse to get a physical address.  Now there are a pair
of functions, arm_devmap_vtop() and arm_devmap_ptov(), to do that.  The
bus_space_map() implementations are rewritten in terms of these.