dt-bindings: reset: rcar-rst: Document r8a77961 support

Add DT binding documentation for the Reset block in the Renesas R-Car
M3-W+ (R8A77961) SoC.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191023122911.12166-4-geert+renesas@glider.be

Documentation/devicetree/bindings/reset/renesas,rst.txt

@@ -31,6 +31,7 @@ Required properties:
 		  - "renesas,r8a7794-rst" (R-Car E2)
 		  - "renesas,r8a7795-rst" (R-Car H3)
 		  - "renesas,r8a7796-rst" (R-Car M3-W)
+		  - "renesas,r8a77961-rst" (R-Car M3-W+)
 		  - "renesas,r8a77965-rst" (R-Car M3-N)
 		  - "renesas,r8a77970-rst" (R-Car V3M)
 		  - "renesas,r8a77980-rst" (R-Car V3H)