rpi_display: add backlight driver and overlay

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>