It seems that the SAI3 UART2 configuration is missing from the pins-imx8mm.h file in the dt-bindings. Can you please take a look at the attached patch and confirm that it should work?
We do not have ported the UART since the pin is already ported in the SAI3_RXC for the UART2_CTS and the SAI3_RXD for the UART2_RTS. These pins are already in use.
I apologize for the inconveniences that this could give.