ACPICA: New: acpi_get_gpe_device interface

This function maps an input GPE index to a GPE block device. Also
Added acpi_current_gpe_count to track the current number of GPEs
that are being managed by the ACPICA core (both FADT-based GPEs
and the GPEs contained in GPE block devices.)

Modify drivers/acpi/system.c to use these 2 new interfaces

Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>