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: NThomas Gleixner <tglx@linutronix.de>
Signed-off-by: NChris Wright <chrisw@sous-sol.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NAndi Kleen <ak@suse.de>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>