ARM: dts: imx6qdl-nitrogen6_max: Remove unneeded unit address

Remove the unneeded unit address for the i2cmux nodes in order to fix
the following build warnings with W=1:

arch/arm/boot/dts/imx6q-nitrogen6_max.dtb: Warning (unit_address_vs_reg): Node /i2cmux@2 has a unit name, but no reg property
arch/arm/boot/dts/imx6q-nitrogen6_max.dtb: Warning (unit_address_vs_reg): Node /i2cmux@3 has a unit name, but no reg property

While at it, rename the i2cmux nodes and subnodes to make it clearer to which
i2c controller the mux belongs to.
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

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

@@ -172,7 +172,7 @@ volume-down {
 		};
 	};
 
-	i2cmux@2 {
+	i2c2mux {
 		compatible = "i2c-mux-gpio";
 		pinctrl-names = "default";
 		pinctrl-0 = <&pinctrl_i2c2mux>;
@@ -183,20 +183,20 @@ i2cmux@2 {
 		i2c-parent = <&i2c2>;
 		idle-state = <0>;
 
-		i2c2@1 {
+		i2c2mux@1 {
 			reg = <1>;
 			#address-cells = <1>;
 			#size-cells = <0>;
 		};
 
-		i2c2@2 {
+		i2c2mux@2 {
 			reg = <2>;
 			#address-cells = <1>;
 			#size-cells = <0>;
 		};
 	};
 
-	i2cmux@3 {
+	i2c3mux {
 		compatible = "i2c-mux-gpio";
 		pinctrl-names = "default";
 		pinctrl-0 = <&pinctrl_i2c3mux>;
@@ -206,7 +206,7 @@ i2cmux@3 {
 		i2c-parent = <&i2c3>;
 		idle-state = <0>;
 
-		i2c3@1 {
+		i2c3mux@1 {
 			reg = <1>;
 			#address-cells = <1>;
 			#size-cells = <0>;