dmaengine: Add STM32 MDMA driver

This patch adds the driver for the STM32 MDMA controller.

Master Direct memory access (MDMA) is used in order to provide high-speed
data transfer between memory and memory or between peripherals and memory.

MDMA controller provides a master AXI interface for main memory and
peripheral registers access (system access port) and a master AHB
interface only for Cortex-M7 TCM memory access (TCM access port).

MDMA works in conjunction with the standard DMA controllers (DMA1 or DMA2).
It offers up to 64 channels, each dedicated to managing memory access
requests from one of the DMA stream memory buffer or other peripherals
(w/ integrated FIFO).

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>