dmaengine: at_hdmac: add missing kernel-doc style description

We get following warning with W=1:

drivers/dma/at_hdmac.c:243: warning: Function parameter or struct member 'boundary' not described in 'at_desc'
drivers/dma/at_hdmac.c:243: warning: Function parameter or struct member 'dst_hole' not described in 'at_desc'
drivers/dma/at_hdmac.c:243: warning: Function parameter or struct member 'src_hole' not described in 'at_desc'
drivers/dma/at_hdmac.c:243: warning: Function parameter or struct member 'memset_buffer' not described in 'at_desc'
drivers/dma/at_hdmac.c:243: warning: Function parameter or struct member 'memset_paddr' not described in 'at_desc'
drivers/dma/at_hdmac.c:243: warning: Function parameter or struct member 'memset_vaddr' not described in 'at_desc'
drivers/dma/at_hdmac.c:255: warning: Enum value 'ATC_IS_PAUSED' not described in enum 'atc_status'
drivers/dma/at_hdmac.c:255: warning: Enum value 'ATC_IS_CYCLIC' not described in enum 'atc_status'
drivers/dma/at_hdmac.c:287: warning: Function parameter or struct member 'cyclic' not described in 'at_dma_chan'
drivers/dma/at_hdmac.c:350: warning: Function parameter or struct member 'memset_pool' not described in 'at_dma'

Fix this by adding the required description and also drop unused struct
member 'cyclic' in 'at_dma_chan'

Reviewed-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Link: https://lore.kernel.org/r/20240130163216.633034-1-vkoul@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>