ps8815: Add displayport related settings
ps8815: Add displayport related settings

This patch adds one more register for displayport related settings
with ps8815.

BUG=b:189587527
BRANCH=firmware-volteer-13672.B-main
TEST=make buildall

Signed-off-by: Devin Lu <052fe648504800b810f29702ef4f3db83fad68bf@quantatw.com>
Change-Id: If79dce87a581923bb1f382786042018bc37c737a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2972021
Reviewed-by: Abe Levkoy <2e1ad7039298f93262cf06bbb457275fb3e2ecb5@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec