default or selected kernelversion does not have config value CONFIG_SERIAL_8250_AU1X00.
Result is shown for kernelversion 5.4.229

Au1x00 serial port support

configname: CONFIG_SERIAL_8250_AU1X00

Linux Kernel Configuration
└─>Device Drivers
└─>Character devices
└─>Serial drivers
└─>Au1x00 serial port support
In linux kernel since version 2.6.20 (release Date: 2007-02-04)  
If you have an Au1x00 SOC based board and want to use the serial port,
say Y to this option. The driver can handle up to 4 serial ports,
depending on the SOC. If unsure, say N.