dmaengine/ste_dma40: fix missing kernel-doc

Missing documentation creates kernel-doc warnings, so add
the documenation.

Signed-off-by: Om Prakash <omprakash.pal@stericsson.com>
Reviewed-by: Rabin Vincent <rabin.vincent@stericsson.com>
Reviewed-by: Jonas Aberg <jonas.aberg@stericsson.com>
Reviewed-by: Srinidhi Kasagar <srinidhi.kasagar@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>