Add PSU callbacks to platforms that use AC power only

This adds PSU callbacks to platforms which run on AC power only.

BUG=chrome-os-partner:23944
BRANCH=none
TEST=tested on beltino, stumpy

Change-Id: I950bae0cb627b62fab08623e47a9bc4b589d8bd3
Reviewed-on: https://chromium-review.googlesource.com/183754
Reviewed-by: David Hendricks <dhendrix@chromium.org>
Tested-by: David Hendricks <dhendrix@chromium.org>
Reviewed-by: Luigi Semenzato <semenzato@chromium.org>
Commit-Queue: David Hendricks <dhendrix@chromium.org>
12 files changed