dt-bindings: watchdog: renesas-wdt: Document r8a77961 support

Document support for the Watchdog Timer in the Renesas R-Car M3-W+
(R8A77961) SoC.

No driver update is needed.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/watchdog/renesas,wdt.txt

@@ -19,6 +19,7 @@ Required properties:
 	         - "renesas,r8a7794-wdt" (R-Car E2)
 	         - "renesas,r8a7795-wdt" (R-Car H3)
 	         - "renesas,r8a7796-wdt" (R-Car M3-W)
+	         - "renesas,r8a77961-wdt" (R-Car M3-W+)
 		 - "renesas,r8a77965-wdt" (R-Car M3-N)
 	         - "renesas,r8a77970-wdt" (R-Car V3M)
 	         - "renesas,r8a77990-wdt" (R-Car E3)