drivers: video: backlight: Register thermal cooling device

Backlight display can be a thermal cooling device which can help reduce
the temperature by reducing the screen brightness. To facilitate this
register the backlight as a cooling device with thermal framework.

In order to register a backlight driver as a cooling device the
devicetree should have the property #cooling-cells defined. backlight
framework looks for this property to register the cooling device.

The backlight framework stores the user brightness request for the
backlight and caps the request based on thermal recommendation before
sending it to the driver. Also, when there is a new cap request the
current brightness will be re-evaluated.

Change-Id: I1405ddd6c3cfff99cd84842d3773851168dcfe78
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>