arm64: dts: rockchip: add missing #msi-cells to rk3399

The rk3399 gic-its was missing the #msi-cells property as found by
dt-schema checks, so add it.
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20190917083625.25818-1-heiko@sntech.de

arch/arm64/boot/dts/rockchip/rk3399.dtsi

@@ -520,6 +520,7 @@ gic: interrupt-controller@fee00000 {
 		its: interrupt-controller@fee20000 {
 			compatible = "arm,gic-v3-its";
 			msi-controller;
+			#msi-cells = <1>;
 			reg = <0x0 0xfee20000 0x0 0x20000>;
 		};