dmaengine: st_fdma: Add STMicroelectronics FDMA DT binding documentation

This patch adds the DT binding documentation for the FDMA constroller
found on STi based chipsets from STMicroelectronics.

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