CONFIG_MDIO_THUNDER is not available for thedefaultarchitecture x86.
Result is shown for architecture 64BIT

ThunderX SOCs MDIO buses

modulename: mdio-thunder.ko


Linux Kernel Configuration
└─>Device Drivers
└─>Network device support
└─>MDIO bus device drivers
└─>PCI support
└─>ThunderX SOCs MDIO buses
In linux kernel since version 3.10 (release Date: 2013-06-30)  
This driver supports the MDIO interfaces found on Cavium
ThunderX SoCs when the MDIO bus device appears as a PCI

source code: