x86/PCI: ACPI based PCI gap calculation

Using ACPI to find free address space allows us to find a gap for the
unallocated PCI resources or MMIO resources for hotplug devices within
the BIOS allowed PCI regions.

It works by evaluating the _CRS object under PCI0 looking for producer
resources.  Then searches the e820 memory space for a gap within these
producer resources.

Signed-off-by: Alok N Kataria <akataria@vmware.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Len Brown <lenb@kernel.org>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>