i386/x8664: cleanup the shared hpet code

Remove hpet_readl/writel from vsyscall.h, where it does not belong
anyway. Use the hpet code itself.

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