default or selected kernelversion does not have config value CONFIG_ARM_AT91_ETHER.
Result is shown for kernelversion 5.9.9

AT91RM9200 Ethernet support

modulename: at91_ether.ko
configname: CONFIG_ARM_AT91_ETHER

Linux Kernel Configuration
└─> Device Drivers
└─> Network device support
└─> Ethernet driver support
└─> AT91RM9200 Ethernet support

If you wish to compile a kernel for the AT91RM9200 and enable
ethernet support, then you should always answer Y to this.

source code: