x86, ioapic, acpi: add a knob to disable IRQ 0 through I/O APIC

As discovered recently some systems exhibit problems when the 8254 timer
IRQ is routed through the I/O APIC.  These problems do not affect the
timer IRQ itself and therefore cannot be detected when the correctness of
operation of the interrupt is verified in check_timer().  Therefore the
I/O APIC path of the timer IRQ has to be disabled entirely.

This is a change that lets platforms ask for the timer IRQ not to be
registered in the I/O APIC interrupt tables.  The local APIC and ExtINTA
paths are unaffected.  This request is only taken into account for ACPI
platforms as MP table systems seem unaffected so far.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: Len Brown <lenb@kernel.org>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>