dmaengine: Add STM32 DMAMUX driver

This patch implements the STM32 DMAMUX driver.

The DMAMUX request multiplexer allows routing a DMA request line between
the peripherals and the DMA controllers of the product. The routing
function is ensured by a programmable multi-channel DMA request line
multiplexer. Each channel selects a unique DMA request line,
unconditionally or synchronously with events from its DMAMUX
synchronization inputs. The DMAMUX may also be used as a DMA request
generator from programmable events on its input trigger signals

Signed-off-by: M'boumba Cedric Madianga <cedric.madianga@gmail.com>
Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>