dmaengine: at_xdmac: add runtime pm support

Add runtime PM support which involves disabling/enabling controller's
clocks on runtime PM suspend/resume ops. The runtime suspend/resume is
done based on the work submitted to the controller: runtime resume is
happening on at_xdmac_start_xfer() and runtime suspend on
at_xdmac_tasklet().
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20221117131547.293044-2-claudiu.beznea@microchip.comSigned-off-by: Vinod Koul <vkoul@kernel.org>