mfd: cros_ec_dev: Register cros_usbpd-charger driver as a subdevice.

Check whether this EC instance has USBPD host command support and
instatiate the cros_usbpd-charger driver as a subdevice in such case.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>