arm64: renesas: r8a7795 dtsi: Add device node for RST module

Add a device node for the RST module, which provides a.o. reset control
and mode pin monitoring.

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