Add bus_dmamap_load_ma() function to load map with the array of
vm_pages.  Provide trivial implementation which forwards the load to
_bus_dmamap_load_phys() page by page.  Right now all architectures use
bus_dmamap_load_ma_triv().

Tested by:	pho (as part of the functional patch)
Sponsored by:	The FreeBSD Foundation
MFC after:	1 month