dt-bindings: stm32-dma: introduce DMA features bitfield

This bitfield intends to address features to be activated within the
driver. Initially the mask was only meant for FIFO Threshold management.
Backward compatibility is preserved but the meaning of this field has been
extended to features instead of only threshold.

Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>