driver core: location: Add "back" as a possible output for panel

Add "back" as a possible panel output when _PLD output from ACPI
indicates back panel.

Fixes: 6423d295 ("driver core: Add sysfs support for physical location of a device")
Signed-off-by: Won Chung <wonchung@google.com>
Link: https://lore.kernel.org/r/20220509214930.3573518-1-wonchung@google.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/physical_location.c

@@ -57,6 +57,9 @@ static ssize_t panel_show(struct device *dev, struct device_attribute *attr,
 	case DEVICE_PANEL_FRONT:
 		panel = "front";
 		break;
+	case DEVICE_PANEL_BACK:
+		panel = "back";
+		break;
 	default:
 		panel = "unknown";
 	}