dmaengine: at_xdmac: use __maybe_unused for pm functions

Use __maybe_unused for pm functions.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20211007111230.2331837-4-claudiu.beznea@microchip.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>