platform/chrome: cros_ec_dev - Register cros-ec sensors

Check whether the ChromeOS Embedded Controller is a sensor hub and in
such case issue a command to get the number of sensors and register them
all.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>