dt-bindings: display: renesas,du: Document cmms property

Document the newly added 'renesas,cmms' property which accepts a list of
phandle and channel index pairs that point to the CMM units available
for each Display Unit output video channel.

Reviewed-by: Rob Herring <robh@kernel.org>
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>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>