mfd: cros_ec: Add helper for event notifier.

Add cros_ec_get_event() entry point to retrieve event within functions
called by the notifier.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Benson Leung <bleung@chromium.org>