refactor(cpufeat): move helpers into .c file, rename FEAT_STATE_

The FEATURE_DETECTION functionality had some definitions in a header
file, although they were only used internally in the .c file.
Move them over there, since there are of no interest to other users.

Also use the opportuntiy to rename the less telling FEAT_STATE_[12]
names, and let the "0" case join the game. We use DISABLED, ALWAYS, and
CHECK now, so that the casual reader has some idea what those numbers
are supposed to mean.

feature_panic() becomes "static inline", since disabling all features
makes it unused, so the compiler complains otherwise.

Finally add a new category "cpufeat" to cover CPU feature related
changes.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: If0c8ba91ad22440260ccff383c33bdd055eefbdc