dt-bindings: watchdog: renesas-wdt: Document r8a774b1 support

RZ/G2N (a.k.a. R8A774B1) watchdog implementation is compatible
with R-Car Gen3, therefore add the relevant documentation.
Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>

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

@@ -10,6 +10,7 @@ Required properties:
 		 - "renesas,r8a7745-wdt" (RZ/G1E)
 		 - "renesas,r8a77470-wdt" (RZ/G1C)
 		 - "renesas,r8a774a1-wdt" (RZ/G2M)
+		 - "renesas,r8a774b1-wdt" (RZ/G2N)
 		 - "renesas,r8a774c0-wdt" (RZ/G2E)
 	         - "renesas,r8a7790-wdt" (R-Car H2)
 	         - "renesas,r8a7791-wdt" (R-Car M2-W)