CONFIG_NIOS2_DTB_PHYS_ADDR is not available for thedefaultarchitecture x86.
Result is shown for architecture nios2

DTB Address

configname: CONFIG_NIOS2_DTB_PHYS_ADDR

Linux Kernel Configuration
└─>Platform options
└─>DTB Address
In linux kernel since version 3.19 (release Date: 2015-02-08)  
Physical address of a dtb blob.