[M252KG] Exclude USB UART from testing
[M252KG] Exclude USB UART from testing

USB UART is dedicated to USB COM and so must exclude from FPGA CI testing.

andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed,kjbracey-arm/mbed,andcor02/mbed-os,kjbracey-arm/mbed,kjbracey-arm/mbed,kjbracey-arm/mbed,mbedmicro/mbed,mbedmicro/mbed