[ACPI] generic Hot Key support

See Documentation/acpi-hotkey.txt

Use cmdline "acpi_specific_hotkey" to enable
legacy platform specific drivers.

http://bugzilla.kernel.org/show_bug.cgi?id=3887

Signed-off-by: Luming Yu <luming.yu@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>

include/acpi/acpi_drivers.h

@@ -108,5 +108,10 @@ int acpi_ec_ecdt_probe (void);
 
 int acpi_processor_set_thermal_limit(acpi_handle handle, int type);
 
+/* --------------------------------------------------------------------------
+                                    Hot Keys
+   -------------------------------------------------------------------------- */
+
+extern int acpi_specific_hotkey_enabled;
 
 #endif /*__ACPI_DRIVERS_H__*/