iio: cros_ec_sensors_core: Add common functions for the ChromeOS EC Sensor Hub.

Add the core functions to be able to support the sensors attached behind
the ChromeOS Embedded Controller and used by other IIO cros-ec sensor
drivers.

The cros_ec_sensor_core driver matches with current driver in ChromeOS
4.4 tree, so it includes all the fixes at the moment. The support for
this driver was made by Gwendal Grignou. The original patch and all the
fixes has been squashed and rebased on top of mainline.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Guenter Roeck <groeck@chromium.org>
[eballetbo: split, squash and rebase on top of mainline the patches
found in ChromeOS tree]
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>