ARM: dts: imx23/28: Remove undocumented "fsl,clkctrl"

Per imx23-clock.yaml and imx28-clock.yaml, fsl,clkctrl is not a valid
compatible string.

Remove it to fix the following dt-schema warning:

imx28-evk.dtb: clkctrl@80040000: compatible: ['fsl,imx28-clkctrl', 'fsl,clkctrl'] is too long
	from schema $id: http://devicetree.org/schemas/clock/imx28-clock.yaml#Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/nxp/mxs/imx23.dtsi

@@ -484,7 +484,7 @@ apbx@80040000 {
 			ranges;
 
 			clks: clkctrl@80040000 {
-				compatible = "fsl,imx23-clkctrl", "fsl,clkctrl";
+				compatible = "fsl,imx23-clkctrl";
 				reg = <0x80040000 0x2000>;
 				#clock-cells = <1>;
 			};

arch/arm/boot/dts/nxp/mxs/imx28.dtsi

@@ -1100,7 +1100,7 @@ apbx@80040000 {
 			ranges;
 
 			clks: clkctrl@80040000 {
-				compatible = "fsl,imx28-clkctrl", "fsl,clkctrl";
+				compatible = "fsl,imx28-clkctrl";
 				reg = <0x80040000 0x2000>;
 				#clock-cells = <1>;
 			};