mfd: cros_ec: Send suspend state notification to EC

Notify EC when going to or returning from suspend so that proper actions
related to wake events can be taken.

Signed-off-by: Shawn Nematbakhsh <shawnn@chromium.org>
Signed-off-by: Thierry Escande <thierry.escande@collabora.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>