ACPI: utils: Add acpi_reduced_hardware() helper

Add a getter for the acpi_gbl_reduced_hardware variable so that modules
can check if they are running on an ACPI reduced-hw platform or not.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>