ARM: dts: r8a7793: Add RWDT node

Add a device node for the Watchdog Timer (WDT) controller on the Renesas
R-Car M2-N (r8a7793) SoC.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>