CONFIG_SOC_RT305X is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

RT305x

modulename: rt305x.ko

configname: CONFIG_SOC_RT305X

Ralink SoC selection
└─>RT305x
In linux kernel since version 3.1 (release Date: 2011-10-24)  
source code:
arch/mips/ralink/rt305x.c