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.