CONFIG_DEBUG_AT91_LAN966_FLEXCOM is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Kernel low-level debugging on LAN966 FLEXCOM USART

configname: CONFIG_DEBUG_AT91_LAN966_FLEXCOM

Kernel low-level debugging port
└─>Kernel low-level debugging on LAN966 FLEXCOM USART
In linux kernel since version 5.2 (release Date: 2019-07-07)  
Say Y here if you want kernel low-level debugging support
on the FLEXCOM port of LAN966.

DEBUG_UART_PHYS | DEBUG_UART_VIRT

0xe0040200 | 0xfd040200 | FLEXCOM0
0xe0044200 | 0xfd044200 | FLEXCOM1
0xe0060200 | 0xfd060200 | FLEXCOM2
0xe0064200 | 0xfd064200 | FLEXCOM3
0xe0070200 | 0xfd070200 | FLEXCOM4

By default, enabling FLEXCOM3 port. Based on requirement, use
DEBUG_UART_PHYS and DEBUG_UART_VIRT configurations from above
table.