arm64: dts: imx8mm-nitrogen-r2: Fix I2C mux subnode name

Per i2c-mux-pca954x.yaml, the I2C subnodes should follow the
'i2c@' format.

Change it to fix the following schema warning:

imx8mm-nitrogen-r2.dtb: i2c-mux@70: Unevaluated properties are not allowed ('i2c3@0' was unexpected)
	from schema $id: http://devicetree.org/schemas/i2c/i2c-mux-pca954x.yaml#



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