[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>
diff --git a/drivers/acpi/ibm_acpi.c b/drivers/acpi/ibm_acpi.c
index 0fb731a..6c8291c 100644
--- a/drivers/acpi/ibm_acpi.c
+++ b/drivers/acpi/ibm_acpi.c
@@ -1185,6 +1185,10 @@
 	if (acpi_disabled)
 		return -ENODEV;
 
+	if (!acpi_specific_hotkey_enabled){
+		printk(IBM_ERR "Using generic hotkey driver\n");
+		return -ENODEV;	
+	}
 	/* these handles are required */
 	if (IBM_HANDLE_INIT(ec,	  1) < 0 ||
 	    IBM_HANDLE_INIT(hkey, 1) < 0 ||