[media] dt-bindings: Document Renesas R-Car FCP power-domains usage

The power domain must be specified to bring the device out of module
standby. Document this in the bindings provided, so that new additions
are not missed.

Signed-off-by: Kieran Bingham <kieran+renesas@bingham.xyz>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>