mfd: cros_ec_dev: Register cros-ec-rtc driver as a subdevice

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

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>