platform/chrome: cros_ec_sysfs: Add cold-ap-off to sysfs reboot.

Add cold-ap-off to ChromeOS EC sysfs reboot file option, corresponds to
the EC_REBOOT_COLD_AP_OFF flag, that will reset EC and keep AP off.

Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Link: https://lore.kernel.org/r/20201221041231.14516-2-pihsun@chromium.org