platform/chrome: cros_ec_commands: Add host command to keep AP off after EC reset.

Add command to EC_CMD_REBOOT_EC to reset EC but don't boot AP.

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/20201221041231.14516-1-pihsun@chromium.org