x86: get irq for hpet timer

HPET timer's IRQ is 0 by default. So we have to select which irq
will be used by these timers. We wait to set the timer's irq until
we really open it in order to reduce the chance of conflicting with
other device.

Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>