i2c: rcar: document R8A77970 bindings

R-Car V3M (R8A77970) SoC also has the R-Car gen3 compatible I2C controller,
so document  the SoC specific bindings.
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

Documentation/devicetree/bindings/i2c/i2c-rcar.txt

@@ -13,6 +13,7 @@ Required properties:
 	"renesas,i2c-r8a7794" if the device is a part of a R8A7794 SoC.
 	"renesas,i2c-r8a7795" if the device is a part of a R8A7795 SoC.
 	"renesas,i2c-r8a7796" if the device is a part of a R8A7796 SoC.
+	"renesas,i2c-r8a77970" if the device is a part of a R8A77970 SoC.
 	"renesas,rcar-gen1-i2c" for a generic R-Car Gen1 compatible device.
 	"renesas,rcar-gen2-i2c" for a generic R-Car Gen2 or RZ/G1 compatible
 				device.