Implement promotions and demotions in the arm64 pmap code. For now we don't
promote memory as I am not sure all the demotion cases are handled, however
it is useful to implement pmap_page_set_memattr. This is used, for example,
when mapping uncached memory for bus_dma(9).

pmap_page_set_memattr needs to demote the DMAP region as on ARM we need to
ensure all mappings to the same physical address have the same attributes.

Reviewed by: kib
Obtained from: ABT Systems Ltd
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D6987