arm64: dts: renesas: spider-cpu: Enable watchdog timer

Enable the watchdog timer on the Spider board.

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/b36b2bb5770e10d906571721a3d73ca205b6f56e.1642525158.git.geert+renesas@glider.be