x86: enable HPET on ICH3 and ICH4

ICH3 and ICH4 have undocumented HPET capabilities. This patch enables
HPET for platforms based around these ICHs.

Tested on various ICH3 and ICH4 platforms.

Because HPET is not officially documented for ICH3/4 and may not have
been validated by chipset folks, we're on thin ice here. I'd recommend
testing this patch in -hrt or -mm for a while and wait for
success/failure reports before feeding it upstream.

tglx: depends on the force_hpet command line option !

Signed-off-by: Udo A. Steinberg <us15@os.inf.tu-dresden.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>