ARM: dts: imx6: rename i2c<n>mux i2c-mux-<n>

Rename i2c<n>mux i2c-mux-<n> to fix below CHECK_DTBS warnings:
arch/arm/boot/dts/nxp/imx/imx6q-nitrogen6_max.dtb: i2c2mux (i2c-mux-gpio): $nodename:0: 'i2c2mux' does not match '^(i2c-?)?mux'
        from schema $id: http://devicetree.org/schemas/i2c/i2c-mux-gpio.yaml

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/nxp/imx/imx6dl-alti6p.dts

@@ -25,7 +25,7 @@
 		clock-output-names = "enet_ref_pad";
 	};
 
-	i2c2-mux {
+	i2c-mux-2 {
 		compatible = "i2c-mux";
 		i2c-parent = <&i2c2>;
 		mux-controls = <&i2c_mux>;
@@ -45,7 +45,7 @@
 		};
 	};
 
-	i2c4-mux {
+	i2c-mux-4 {
 		compatible = "i2c-mux";
 		i2c-parent = <&i2c4>;
 		mux-controls = <&i2c_mux>;

arch/arm/boot/dts/nxp/imx/imx6qdl-nitrogen6_max.dtsi

@@ -124,7 +124,7 @@
 		};
 	};
 
-	i2c2mux {
+	i2c-mux-2 {
 		compatible = "i2c-mux-gpio";
 		pinctrl-names = "default";
 		pinctrl-0 = <&pinctrl_i2c2mux>;
@@ -148,7 +148,7 @@
 		};
 	};
 
-	i2c3mux {
+	i2c-mux-3 {
 		compatible = "i2c-mux-gpio";
 		pinctrl-names = "default";
 		pinctrl-0 = <&pinctrl_i2c3mux>;