arm64: dts: renesas: r8a77970: Add RPC-IF support

Describe RPC-IF in the R8A77970 device tree.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Link: https://lore.kernel.org/r/ba8bb326-7e77-6ab7-668f-fdc22010c8ef@cogentembedded.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>