ARM: dts: renesas: Add RPC node to R8A779A0 V3U

The R-Car V3U does support RPC interface, however the support for it is
missing in upstream Linux DTs as of commit 9f4ad9e425a1 ("Linux 5.12"),
add the node into u-boot.dtsi to let U-Boot access the SPI NOR or HF.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>