[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>

Documentation/devicetree/bindings/media/renesas,fcp.txt

@@ -23,6 +23,10 @@ are paired with. These DT bindings currently support the FCPV and FCPF.
  - reg: the register base and size for the device registers
  - clocks: Reference to the functional clock
 
+Optional properties:
+ - power-domains : power-domain property defined with a power domain specifier
+		   to respective power domain.
+
 
 Device node example
 -------------------
@@ -31,4 +35,5 @@ Device node example
 		compatible = "renesas,r8a7795-fcpv", "renesas,fcpv";
 		reg = <0 0xfea2f000 0 0x200>;
 		clocks = <&cpg CPG_MOD 602>;
+		power-domains = <&sysc R8A7795_PD_A3VP>;
 	};