RF_CACHEABLE flag

[Justin Hibbits]

The Freescale/NXP Datapath Acceleration Architecture uses both
cache-inhibited and cache-enabled memory regions for buffer portals.
This doesn't quite fit right into the existing framework, so I've
added to my personal repo (on github) a RF_CACHEABLE flag to be used
by this.  Now that I'm ready to commit the driver to head, I want to
float this on -arch to get opinions.

I did consider another route, using bus_space_map()/bus_space_unmap(),
and stashing sizes around, but adding a simple flag to rman would take
care of all the details, and rman already knows all the other details
for the region anyway.

I put the diff on phabricator, at https://reviews.freebsd.org/D5384 .

Thoughts on this?

- Justin