iio / platform: cros_ec: Add cros-ec-sensorhub driver

Similar to HID sensor stack, the new driver sits between cros-ec-dev
and the IIO device drivers:

The EC based IIO device topology would be:

iio:device1 ->
   ...0/0000:00:1f.0/PNP0C09:00/GOOG0004:00/cros-ec-dev.6.auto/
                                            cros-ec-sensorhub.7.auto/
                                            cros-ec-accel.15.auto/
                                            iio:device1

It will be expanded to control EC sensor FIFO.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
[Fix "unknown type name 'uint32_t'" type errors]
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>