ARM: dts: renesas: Disable RPC driver on R8A779H0 V4M Gray Hawk board

ARM: dts: renesas: Disable RPC driver on R8A779H0 V4M Gray 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.

Signed-off-by: Nguyen Tran <nguyen.tran.pz@bp.renesas.com>
Reviewed-by: Khanh Le <khanh.le.xr@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.]