ARM: dts: renesas: Disable RPC driver on R8A779G0 V4H White Hawk board

As requirement of CR side, QSPI Flash usage via RPC driver shall
be disabled and leaving the control of this module to CR side.
Perform DT modification to disable the RPC SPI.

Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Signed-off-by: Cong Dang <cong.dang.xn@renesas.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
[Marek: Do not modify defconfig, modify the DT instead, this way
        the RPC SPI can be enabled without recompiling the U-Boot
	itself. Update commit message accordingly.]