arm64: dts: renesas: r8a774a1: Add RWDT node

Add a device node for the Watchdog Timer (RWDT) controller on the Renesas
RZ/G2M (r8a774a1) SoC.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>