Implement pmap_change_attr() for PowerPC (Book-E only for now)

Summary:
Some drivers need special memory requirements. X86 solves this with a
pmap_change_attr() API, which DRM uses for changing the mapping of the GART and
other memory regions. Implement the same function for PowerPC. AIM currently
does not need this, but will in the future for DRM, so a default is added for
that, for business as usual. Book-E has some drivers coming down that do
require non-default memory coherency. In this case, the Datapath Acceleration
Architecture (DPAA) based ethernet controller has 2 regions for the buffer
portals: cache-inhibited, and cache-enabled. By default, device memory is
cache-inhibited. If the cache-enabled memory regions are mapped
cache-inhibited, an alignment exception is thrown on access.

Test Plan:
Tested with a new driver to be added after this (DPAA dTSEC ethernet driver).
No alignment exceptions thrown, driver works as expected with this.

Reviewed By: nwhitehorn
Sponsored by: Alex Perez/Inertial Computing
Differential Revision: https://reviews.freebsd.org/D5471