CONFIG_SOC_MT7621 is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

MT7621

modulename: mt7621.ko

configname: CONFIG_SOC_MT7621

Ralink SoC selection
└─>MT7621
In linux kernel since version 3.10 (release Date: 2013-06-30)  
The MT7621 system-on-a-chip includes an 880 MHz MIPS1004Kc
dual-core CPU, a 5-port 10/100/1000 switch/PHY and one RGMII.

source code:
/arch/mips/ralink/mt7621.c