mfd: google,cros-ec: add DT bindings for a baseboard's switch device

This is for ChromeOS tablets which have a 'cros_cbas' switch device
in the "Whiskers" base board. This device can be instantiated only by
device tree on ARM platforms. ChromeOS EC doesn't provide a way to
probe the device.

Signed-off-by: Ikjoon Jang <ikjn@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>