arm64: dts: renesas: r8a77965: Add Watchdog Timer controller node using RCLK Watchdog Timer

Add a device node for the Watchdog Timer (WDT) controller on the Renesas
R-Car M3-N (R8A77965) SoC.

Based on a similar patch of the R8A7796 device tree
by Geert Uytterhoeven <geert+renesas@glider.be>.

Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>