ASoC: rsnd: Document r8a77470 bindings

Document SoC specific bindings for R-Car RZ/G1C(r8a77470) SoC.
Signed-off-by: Cao Van Dong <cv-dong@jinso.co.jp>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/renesas,rsnd.txt

@@ -266,6 +266,7 @@ Required properties:
 				    - "renesas,rcar_sound-r8a7743" (RZ/G1M)
 				    - "renesas,rcar_sound-r8a7744" (RZ/G1N)
 				    - "renesas,rcar_sound-r8a7745" (RZ/G1E)
+				    - "renesas,rcar_sound-r8a77470" (RZ/G1C)
 				    - "renesas,rcar_sound-r8a774a1" (RZ/G2M)
 				    - "renesas,rcar_sound-r8a774c0" (RZ/G2E)
 				    - "renesas,rcar_sound-r8a7778" (R-Car M1A)