media: dt-bindings: Add bindings for Raspberry Pi PiSP Back End

Add bindings for the Raspberry Pi PiSP Back End memory-to-memory image
signal processor.

Datasheet:
https://datasheets.raspberrypi.com/camera/raspberry-pi-image-signal-processor-specification.pdf



Signed-off-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Naushir Patuck <naush@raspberrypi.com>