arm64: dts: zynqmp: Add missing gpio-controller to ps gpio

Add missing gpio-controller property to ps gpio.
This was found via DT schema validation.
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/arm64/boot/dts/xilinx/zynqmp.dtsi

@@ -410,6 +410,7 @@ gpio: gpio@ff0a0000 {
 			compatible = "xlnx,zynqmp-gpio-1.0";
 			status = "disabled";
 			#gpio-cells = <0x2>;
+			gpio-controller;
 			interrupt-parent = <&gic>;
 			interrupts = <0 16 4>;
 			interrupt-controller;