default or selected kernelversion does not have config value CONFIG_NET_MEDIATEK_MDIO_MT7620.
Result is shown for kernelversion 6.9.9

CONFIG_NET_MEDIATEK_MDIO_MT7620 (not configurable)

configname: CONFIG_NET_MEDIATEK_MDIO_MT7620

Linux Kernel Configuration
└─>Device Drivers
└─>Staging drivers
└─>NET_MEDIATEK_MDIO_MT7620
In linux kernel since version 4.2 (release Date: 2015-08-30)