ARM: dts: r8a7779: Add device node for RESET/WDT module

Add a device node for the RESET/WDT module, which provides a.o. reset
control, mode pin monitoring, and watchdog control.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Dirk Behme <dirk.behme@de.bosch.com>