CONFIG_NIOS2_KERNEL_MMU_REGION_BASE is not available for thedefaultarchitecture x86.
Result is shown for architecture nios2

Virtual base address of the kernel MMU region

configname: CONFIG_NIOS2_KERNEL_MMU_REGION_BASE

Linux Kernel Configuration
└─>Advanced setup
└─>Virtual base address of the kernel MMU region
In linux kernel since version 3.19 (release Date: 2015-02-08)  
This option allows you to set the virtual base address of the kernel MMU region.