cros_ec: Move cros_ec_dev module to drivers/mfd

The cros_ec_dev module is responsible for registering the MFD devices
attached to the ChromeOS EC. This patch moves this module to drivers/mfd
so calls to mfd_add_devices() are not done from outside the MFD subtree
anymore.

Signed-off-by: Thierry Escande <thierry.escande@collabora.com>
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
Tested-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>