arm64: dts: renesas: r8a77970: Add RWDT node

Add a device node for the Watchdog Timer (WDT) controller on the
Renesas R-Car V3M (r8a77970) SoC.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>