rtc: cros-ec: add cros-ec-rtc driver.

On platforms with a Chrome OS EC, the EC can function as a simple RTC.
Add a basic driver with this functionality.

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