arm64: enable superpage mappings by pmap_mapdev{,_attr}()

In order for pmap_kenter{,_device}() to create superpage mappings,
either 64 KB or 2 MB, pmap_mapdev{,_attr}() must request appropriately
aligned virtual addresses.

Reviewed by: markj
Tested by: gallatin
Differential Revision: https://reviews.freebsd.org/D42737