x86: hpet: modify IXP400 quirk to enable interrupts

The current quirk is incomplete. Some more chipset fiddling has to be
done to enable HPET interrupts. This patch aims to do this. From my
tests it seems to work faultlessly.

But the official statement is that HPET is not supported on SB4X0.

Users will still have to use hpet=force to enable it.

Use it at your own risk.

Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>