dt-bindings: display: renesas,cmm: Add R-Car CMM documentation

Add device tree bindings documentation for the Renesas R-Car Display
Unit Color Management Module.

CMM is the image enhancement module available on each R-Car DU video
channel on R-Car Gen2 and Gen3 SoCs (V3H and V3M excluded).

Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>