x86: hpet: Cleanup the clockevents init and register code

No need to recalculate the frequency and the conversion factors over
and over. Calculate the frequency once and use the new config/register
interface and let the core code do the math.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <john.stultz@linaro.org>
Reviewed-by: Ingo Molnar <mingo@elte.hu>
Link: http://lkml.kernel.org/r/%3C20110518210136.646482357%40linutronix.de%3E