mfd: cros_ec: Introduce RTC commands and events definitions.

The EC can function as a simple RT, this patch adds the RTC related
definitions needed by the rtc-cros-ec driver.

Signed-off-by: Stephen Barber <smbarber@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Benson Leung <bleung@chromium.org>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>