platform/chrome: Add ChromeOS EC ISHTP driver

This driver implements a slim layer to enable the ChromeOS
EC kernel stack (cros_ec) to communicate with ChromeOS EC
firmware running on the Intel Integrated Sensor Hub (ISH).

The driver registers a ChromeOS EC MFD device to connect
with cros_ec kernel stack (upper layer), and it registers a
client with the ISH Transport Protocol bus (lower layer) to
talk with the ISH firwmare. See description of the ISHTP
protocol at Documentation/hid/intel-ish-hid.txt

Signed-off-by: Rushikesh S Kadam <rushikesh.s.kadam@intel.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Reviewed-by: Jett Rink <jettrink@chromium.org>
Tested-by: Jett Rink <jettrink@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>