Add the ACPI_PWR_FOR_SLEEP method. It takes a device and outputs the

appropriate power (Dx) state, if the BIOS suggests one.

MFC after:	3 weeks

sys/dev/acpica/acpi_if.m

@@ -108,6 +108,26 @@ METHOD ACPI_STATUS evaluate_object {
 	ACPI_BUFFER	*ret;
 };
 
+#
+# Get the highest power state (D0-D3) that is usable for a device when
+# suspending/resuming.  If a bus calls this when suspending a device, it
+# must also call it when resuming.
+#
+# device_t bus:  parent bus for the device
+#
+# device_t dev:  check this device's appropriate power state
+#
+# int *dstate:  if successful, contains the highest valid sleep state
+#
+# Returns:  0 on success, ESRCH if device has no special state, or
+#   some other error value.
+#
+METHOD int pwr_for_sleep {
+	device_t	bus;
+	device_t	dev;
+	int		*dstate;
+};
+
 #
 # Rescan a subtree and optionally reattach devices to handles.  Users
 # specify a callback that is called for each ACPI_HANDLE of type Device