dt-bindings: stm32-dma: add a property to handle STM32 DMAMUX

STM32 DMA controller has to exposed its number of request line to be
addressed via STM32 DMAMUX.

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