arm64: dts: renesas: r8a779g0: Add RWDT node

Add a device node for the RCLK Watchdog Timer (RWDT) on the Renesas
R-Car V4H (R8A779G0) SoC.

Based on a patch in the BSP by Thanh Quan.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/187904c279be6654ea3deb11b7250b64dd18c3b5.1662715538.git.geert+renesas@glider.be