mfd: cros_ec_dev: Load lightbar module only present

The pixel lightbar is only present on chromebook pixel (link), pixel 2
and pixel C. For the latter two, the EC reports its presence.
Instead of always loading the lightbar driver on all chromebook, only
load it when reported by the EC or Link device.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Acked-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Tested-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>