If the HW Reduced ACPI mode bit is set in the FADT, ACPICA uses
the optional sleep control and sleep status registers for making
the system enter sleep states (including S5), so it is not possible
to use system sleep states or power it off using ACPI if the HW
Reduced ACPI mode bit is set and those registers are not available.

For this reason, add a new function, acpi_sleep_state_supported(),
checking if the HW Reduced ACPI mode bit is set and whether or not
system sleep states are usable in that case in addition to checking
the return value of acpi_get_sleep_type_data() and make the ACPI
sleep setup routines use that function to check the availability of
system sleep states.

Among other things, this prevents the kernel from attempting to
use ACPI for powering off HW Reduced ACPI systems without the sleep
control and sleep status registers, because ACPI power off doesn't
have a chance to work on them.  That allows alternative power off
mechanisms that may actually work to be used on those systems.  The
affected machines include Dell Venue 8 Pro, Asus T100TA, Haswell
Desktop SDP and Ivy Bridge EP Demo depot.

References: https://bugzilla.kernel.org/show_bug.cgi?id=70931Reported-by: Adam Williamson <awilliam@redhat.com>
Tested-by: Aubrey Li <aubrey.li@linux.intel.com>
Cc: 3.4+ <stable@vger.kernel.org> # 3.4+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>