ACPI: Add support to parse 2nd MADT

When a BIOS bug presents multiple APIC/MADTs,
Linux currently uses the 1st and ignores the 2nd.

But some machines work better if we use the 2nd.

http://bugzilla.kernel.org/show_bug.cgi?id=7465

Add a warning and boot parameter "acpi_apic_instance=2"
to allow parsing the 2nd.

No change to default behaviour in this patch.

Signed-off-by: Len Brown <len.brown@intel.com>