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>