CONFIG_DEBUG_MMRS is not available for thedefaultarchitecture x86.
Result is shown for architecture blackfin

Generate Blackfin MMR tree

configname: CONFIG_DEBUG_MMRS

Linux Kernel Configuration
└─>Kernel hacking
└─>Generate Blackfin MMR tree
In linux kernel since version 2.6.24 (release Date: 2008-01-24)  
Create a tree of Blackfin MMRs via the debugfs tree. If
you enable this, you will find all MMRs laid out in the
/sys/kernel/debug/blackfin/ directory where you can read/write
MMRs directly from userspace. This is obviously just a debug
feature.

selects
CONFIG_DEBUG_FS