ASoC: rsnd: Document r8a7778-specific binding

Add the missing r8a7778-specific compatible value, which is already in
use since v4.1-rc1.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@kernel.org>

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

@@ -5,6 +5,7 @@ Required properties:
 				  "renesas,rcar_sound-gen1" if generation1, and
 				  "renesas,rcar_sound-gen2" if generation2
 				  Examples with soctypes are:
+				    - "renesas,rcar_sound-r8a7778" (R-Car M1A)
 				    - "renesas,rcar_sound-r8a7790" (R-Car H2)
 				    - "renesas,rcar_sound-r8a7791" (R-Car M2-W)
 - reg				: Should contain the register physical address.