dmaengine: imx-sdma: Add missing struct documentation

Fix compile warning that 'Function parameter or member not described'
with 'W=1' option:

Add missing description for struct sdma_desc

There is not any description for struct sdma_script_start_addrs,
so use /* instead of /**

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Link: https://lore.kernel.org/r/1657086309-7964-1-git-send-email-shengjiu.wang@nxp.com


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