baytrail: add dunit access and registers

The dunit on baytrail is the dram unit. Provide a means
to access the configuration registers there using the
proper IOSF mechanisms.

BUG=chrome-os-partner:22875
BRANCH=none
TEST=Built and booted. Able to read dram registers.

Change-Id: I4d5c019720a7883fe93f3e1860bcd57ce2ea6542
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/170490