dmaengine: st_fdma: Add STMicroelectronics FDMA driver header file

This header file will also be used by the dma xbar driver in the
future.

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>