i386: prepare sharing the hpet code with x86_64

The hpet implementations of i386 and x8664 has been mostly the same
before the clock events conversion of i386. The clock events
conversion of i386 hpet is already done. So it makes sense to share
the code for the x86_64 clock events conversion.

Abstract out the mapping functions.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>