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>