Basically just start with what you’re aiming to enable and work backwards (as you’ve started to do). With judicious use of grep find out where that symbol is defined. If it’s in arch configs for other arches but not your own, it’s probably that.
There may be better tools out there to do this, but in my experience just sleuthing it out a bit will answer your question. The Kconfig system can be complex, but the files are pretty readable.
I have a couple of very minor commits in Linux and, in the 3.0 era, had my name at the top of a source file for a platform that never saw the light of day and was later removed wholesale.
Still feel that invisible feather in my cap.