default or selected kernelversion does not have config value CONFIG_AB8500_DEBUG.
Result is shown for kernelversion 6.9.9

Enable debug info via debugfs

modulename: ab8500-debugfs.ko

configname: CONFIG_AB8500_DEBUG

Linux Kernel Configuration
└─>Device Drivers
└─>Multifunction device drivers
└─>Enable debug info via debugfs
In linux kernel since version 2.6.37 (release Date: 2011-01-04)  
Select this option if you want debug information using the debug
filesystem, debugfs.

source code:
drivers/mfd//ab8500-debugfs.c