watchdog: renesas-wdt: Add support for the r8a7796 wdt

Document support for the Watchdog Timer (WDT) Controller in the Renesas
R-Car M3-W (r8a7796) SoC.

No driver update is needed.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

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

 Renesas Watchdog Timer (WDT) Controller
 
 Required properties:
-- compatible : Should be "renesas,r8a7795-wdt", or "renesas,rcar-gen3-wdt"
+- compatible : Should be "renesas,<soctype>-wdt", and
+	       "renesas,rcar-gen3-wdt" as fallback.
+	       Examples with soctypes are:
+	         - "renesas,r8a7795-wdt" (R-Car H3)
+	         - "renesas,r8a7796-wdt" (R-Car M3-W)
 
   When compatible with the generic version, nodes must list the SoC-specific
   version corresponding to the platform first, followed by the generic