x86/hpet: setup HPET even when disabled due to stopping in deep C states

x86/hpet: setup HPET even when disabled due to stopping in deep C states

Always allow the HPET to be setup, but don't report a frequency back
to the platform time source probe in order to avoid it from being
selected as a valid timer if it's not usable.

Doing the setup even when not intended to be used as a platform timer
is required so that is can be used in legacy replacement mode in order
to assert the IO-APIC is capable of receiving interrupts.

Fixes: c12731493a ('x86/hpet: Use another crystalball to evaluate HPET usability')
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>