arm64: dts: renesas: r8a779f0: Add RWDT node

Add a device node for the RCLK Watchdog Timer (RWDT) on the Renesas
R-Car S4-8 (R8A779F0) SoC.

Extracted from a larger patch in the BSP by LUU HOAI.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/556a7f41bdadceecbe8b59b79ac7e9f592ca17a2.1642525158.git.geert+renesas@glider.be