Either a 8-second press on the power button or the XPSHOLD signal going
down (triggered the AP/PMIC shutdown sequence) will trigger a shutdown
of the system and switch off all power rails.
Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
BUG=None
TEST=on Discovery, test manually various power key press duration and
see power on/power off traces.
Change-Id: Ic37e4a51fe6b131034e24c55f05dae8119ce9992