extres/clk: Improve sysctl dump of clocks.

Always recalculate the frequency, the cache is lazily initialized so it is not always up to date.
While I'm in mark sysctl as MPSAFE.

Discussed with: manu, adrian
MFC after: 1 week

(cherry picked from commit 72a2f3b5e28ada60de01f08b28888f70eec0baed)