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>