dmaengine: at_hdmac: Use pm_ptr()

Use pm_ptr() macro to fill at_dma_driver.driver.pm. In case CONFIG_PM is
not enabled, the macro will return NULL. When NULL, at_dma_dev_pm_ops will
end up being unused, so prepend it with the __maybe_unused attribute.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20221025090306.297886-1-tudor.ambarus@microchip.com
Link: https://lore.kernel.org/r/20221025090306.297886-26-tudor.ambarus@microchip.com


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