arm64: dts: imx8mq-nitrogen: Fix PCA9546 I2C subnodes

Per i2c-mux-pca954x.yaml, the PCA9546 I2C subnodes should be in the
'i2c@' format.

Change it accordingly to fix the following schema warning:

imx8mm-nitrogen-r2.dtb: i2c-mux@70: Unevaluated properties are not allowed ('i2c3@0' was unexpected)

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