watchdog: renesas-wdt: Add support for the R8A77965 WDT

Document support for the Watchdog Timer (WDT) Controller in the Renesas
R-Car M3-N (R8A77965) SoC. No driver update is needed.

Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
[wsa: rebased to v4.17-rc3]
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>