mfd: cros_ec: Instantiate sub-devices from device tree

If the EC device tree node has sub-nodes, try to instantiate them as
MFD sub-devices.  We can configure the EC features provided by the board.

Signed-off-by: Todd Broch <tbroch@chromium.org>
Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>