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>
Link: https://lore.kernel.org/r/20201007083113.567559-6-vkoul@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>