arm64: dts: imx8mm-nitrogen-r2: Pass the i2c3 unit name

Pass the i2c3 unit name to fix the following W=1 build warning:

arch/arm64/boot/dts/freescale/imx8mm-nitrogen-r2.dts:159.8-172.5: Warning (unit_address_vs_reg): /soc@0/bus@30800000/i2c@30a40000/i2cmux@70/i2c3: node has a reg or ranges property, but no unit name

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>