CHROMIUM: drm/anx7808: Add Aux channel reading.

CHROMIUM: drm/anx7808: Add Aux channel reading.

Adds register writing functions and helper functions for setting and clearing
bits.

Adds aux channel read function.

Adds DPCD read function.

Adds a code to print the firmware version of an HDMI dongle, when debugging is
on.

BUG=chrome-os-partner:16196
TEST=With verbose logging, see "ANX7730 Firmware version 0x0d."

Change-Id: I71f0778b9226685d0645234466517818330e61f3
Signed-off-by: Jeremy Thorpe <jeremyt@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/48861