Map device memory using PTE_DEVICE attributes, and also ensure that the
shared flag is set on normal-memory mappings made via pmap_kenter() for SMP.

The "shared flag" part of this change isn't obvious from the diff, here's
the deal... by using the array of preformatted page table entry templates
instead of constructing the PTE from scratch, we automatically get the
right attribute bits set for both caching and shared.

MFC after:	1 week