Add a mailbox-driven backlight controller for the Raspberry Pi DSI
touchscreen display. Requires updated GPU firmware to recognise the
mailbox request.
Signed-off-by: Gordon Hollingworth <gordon@raspberrypi.org>
Add Raspberry Pi firmware driver to the dependencies of backlight driver
Otherwise the backlight driver fails to build if the firmware
loading driver is not in the kernel
Signed-off-by: Alex Riesen <alexander.riesen@cetitec.com>