MFC r244414:
Properly implement pmap_[get|set]_memattr

MFC r244466:
Create an architecture-agnostic buffer pool manager that uses uma(9) to
manage a set of power-of-2 sized buffers for bus_dmamem_alloc().

MFC r244471:
Busdma enhancements, especially for managing small uncacheable buffers.

MFC r244743:
Use C comments instead of C++ comments.

MFC r244474:
We want to build subr_busdma_bufalloc.c on arm.

Submitted by:	cognet
Approved by:	marius
Obtained from:	Netflix