ARM: dts: imx6ul: Add clock and PGC node to GPC

According to fsl,imx-gpc.yaml the General Power Control requires
a ipg clock and a Power Gating Control node. So add them to fix
the dtbs_check warnings on i.MX6UL boards:

gpc@20dc000: 'clocks' is a required property
gpc@20dc000: 'clock-names' is a required property
gpc@20dc000: 'pgc' is a required property
Suggested-by: Jacky Bai <ping.bai@nxp.com>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6ul.dtsi

@@ -719,6 +719,18 @@ gpc: gpc@20dc000 {
 				#interrupt-cells = <3>;
 				interrupts = <GIC_SPI 89 IRQ_TYPE_LEVEL_HIGH>;
 				interrupt-parent = <&intc>;
+				clocks = <&clks IMX6UL_CLK_IPG>;
+				clock-names = "ipg";
+
+				pgc {
+					#address-cells = <1>;
+					#size-cells = <0>;
+
+					power-domain@0 {
+						reg = <0>;
+						#power-domain-cells = <0>;
+					};
+				};
 			};
 
 			iomuxc: pinctrl@20e0000 {