dmaengine: dma40: add signal documentation to the device tree bindings

The DMA40 device tree documentation was vague on the second cell passed
in the configuration node for consumers, and did not specify what the
available signals were connected to. Extend the documentation with this
information for the DB8500 ASIC.

Reported-by: Pawel Kulakowski <pawel.kulakowski@tieto.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>