dmaengine: owl-dma: fix kernel-doc style for enum

Driver doesn't use keyword enum for enum owl_dmadesc_offsets resulting
in warning:

drivers/dma/owl-dma.c:139: warning: cannot understand function prototype: 'enum owl_dmadesc_offsets '

So add the keyword to fix it and also add documentation for missing OWL_DMADESC_SIZE

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