CONFIG_NIOS2 is not available for thedefaultarchitecture x86.
Result is shown for architecture nios2

CONFIG_NIOS2 (not configurable)

configname: CONFIG_NIOS2

Linux Kernel Configuration
└─>NIOS2
In linux kernel since version 3.10 (release Date: 2013-06-30)  

is selected by
CONFIG_SMC91X