kirby: Pull XPSHOLD low to shut down the AP
kirby: Pull XPSHOLD low to shut down the AP

On Kirby, the EC has no direct control over several power rails. To shut
down the AP, we need to pull low XPSHOLD from the EC so that PMIC would
turn off AP power.

BUG=chrome-os-partner:22101
TEST=Repeatedly do 'power on' and 'power off'.
TEST=Reboot the EC and see AP booted.
BRANCH=None

Change-Id: I21d04e46a6cfb455ced073389a928f2549d5212d
Signed-off-by: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/167201
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

thehobn/ec,fourier49/BIZ_EC,md5555/ec,md5555/ec,mtk09422/chromiumos-platform-ec,mtk09422/chromiumos-platform-ec,eatbyte/chromium-ec,alterapraxisptyltd/chromium-ec,alterapraxisptyltd/chromium-ec,thehobn/ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,md5555/ec,eatbyte/chromium-ec,longsleep/ec,alterapraxisptyltd/chromium-ec,fourier49/BZ_DEV_EC,coreboot/chrome-ec,coreboot/chrome-ec,thehobn/ec,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,alterapraxisptyltd/chromium-ec,coreboot/chrome-ec,fourier49/BZ_DEV_EC,longsleep/ec,fourier49/BIZ_EC,longsleep/ec,fourier49/BIZ_EC,thehobn/ec,akappy7/ChromeOS_EC_LED_Diagnostics,eatbyte/chromium-ec,gelraen/cros-ec,mtk09422/chromiumos-platform-ec,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BZ_DEV_EC,coreboot/chrome-ec,fourier49/BIZ_EC,fourier49/BZ_DEV_EC,gelraen/cros-ec,gelraen/cros-ec,md5555/ec,mtk09422/chromiumos-platform-ec,gelraen/cros-ec,eatbyte/chromium-ec,longsleep/ec,akappy7/ChromeOS_EC_LED_Diagnostics