arm64: dts: rockchip: rk356x: Move PCIe MSI to use GIC ITS instead of MBI

Rockchip 356x device-tree now supports GIC ITS. Move PCIe controller's
MSI to use ITS instead of MBI. This removes extra CPU overhead of handling
PCIe MBIs by letting GIC's ITS to serve the PCIe MSIs.

Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>