x86, apic: Add probe() for apic_flat

x86, apic: Add probe() for apic_flat

Currently we start with the default apic_flat mode and switch to some other
apic model depending on the apic drivers acpi_madt_oem_check() routines and
later followed by the apic drivers probe() routines.

Once we selected non flat mode there was no case where we fall back to
flat mode again.

Upcoming changes allow bios-enabled x2apic mode to be disabled by the OS
if interrupt-remapping etc is not setup properly by the bios.

We now has a case for the apic to fall back to legacy flat mode during
apic driver probe() seqeuence. Add a simple flat_probe() which allows
the apic_flat mode to be the last fallback option.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/20111222014632.484984298@sbsiddha-desk.sc.intel.com
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>