CHROMIUM: auxdisplay/ps8620: implement backlight control

CHROMIUM: auxdisplay/ps8620: implement backlight control

Add a standard backlight interface to the PS8622 bridge.

Also refactor how we are storing the driver data in a more classical fashion.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
BUG=chrome-os-partner:17227
TEST=on Spring, boot ChromeOS and press Backlight+/- keys from 100% to 0%
and back.

Change-Id: I1989192161ed6f6d5aafb5399658114a2f60bd97
Reviewed-on: https://gerrit.chromium.org/gerrit/43307
Reviewed-by: Todd Broch <tbroch@chromium.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Commit-Queue: Vincent Palatin <vpalatin@chromium.org>
Tested-by: Vincent Palatin <vpalatin@chromium.org>