default or selected kernelversion does not have config value CONFIG_AS21XXX_PHY.
Result is shown for kernelversion 6.16-rc6

Aeonsemi AS21xxx PHYs

modulename: as21xxx.ko

configname: CONFIG_AS21XXX_PHY

Linux Kernel Configuration
└─>Device Drivers
└─>Network device support
└─>PHY Device support and infrastructure
└─>Aeonsemi AS21xxx PHYs
In linux kernel since version 6.1.145 (release Date: 2025-07-14)  
Currently supports the Aeonsemi AS21xxx PHY.

These are C45 PHYs 10G that require all a generic firmware.

Supported PHYs AS21011JB1, AS21011PB1, AS21010JB1, AS21010PB1,
AS21511JB1, AS21511PB1, AS21510JB1, AS21510PB1, AS21210JB1,
AS21210PB1 that all register with the PHY ID 0x7500 0x7500
before the firmware is loaded.

source code: