arm64: dts: renesas: r8a77990: ebisu: Enable watchdog timer

Add a device node for the Watchdog Timer (WDT) controller on the
R8A77990 SoC, and enable the watchdog on the Ebisu board.

Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
[geert: Squashed 2 commits]
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>