x86: share hpet.h with i386

hpet.h in asm-i386 and asm-x86_64 contain tons of duplicated stuff.
Consolidate into one shared header file.

AK: Fix i386 compilation with !X86_IO_APIC

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: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>