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