doc: Clean up documentation for cycle counter API

Discussion in #55658 showed that our API docs for the hardware cycle
counter layer were a little lacking.  Technically it's owned by the
architecture layer, but the arch docs were unhelpful.  And the arch
layers uniformly farm it out to an API in the timer drivers that was
never documented at all.  Clean things up.

Signed-off-by: Andy Ross <andyross@google.com>