[M] Change in coreboot[main]: cpu/x86/mtrr: Use fls/ffs from lib.h

[Angel Pons (Code Review)]

Attention is currently required from: Angel Pons, Felix Held, Fred Reitberger, Jason Glenesk, Jérémy Compostella, Matt DeVillier, Shuo Liu.

Angel Pons has posted comments on this change by Shuo Liu. ( https://review.coreboot.org/c/coreboot/+/85104?usp=email )

Change subject: cpu/x86/mtrr: Use fls/ffs from lib.h
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Patch Set 5: Code-Review+2

(1 comment)

Commit Message:

PS5: 
Making a note for myself because these three-letter abbreviations are obviously clear and totally not confusing at all:

```
__fls = fms
__ffs = fls
```