dt-bindings: dma: st-stm32-dmamux: Add description for dma-cell values

The dma-cell values for the stm32-dmamux are used to craft the DMA spec
for the actual controller. These values are currently undocumented
leaving the user to reverse engineer the driver in order to determine
their meaning. Add a basic description, while avoiding duplicating
information by pointing the user to the associated DMA docs that
describe the fields in depth.

Signed-off-by: Ken Sloat <ksloat@cornersoftsolutions.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Link: https://lore.kernel.org/r/20241206115018.1155149-1-ksloat@cornersoftsolutions.com


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