dt-bindings: spi: stm32-qspi: add dma properties

This patch adds description of dma properties (optional).
Signed-off-by: NLudovic Barre <ludovic.barre@st.com>
Signed-off-by: NMark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-stm32-qspi.txt

@@ -19,8 +19,11 @@ Required properties:
 - reg: chip-Select number (QSPI controller may connect 2 flashes)
 - spi-max-frequency: max frequency of spi bus
 
-Optional property:
+Optional properties:
 - spi-rx-bus-width: see ./spi-bus.txt for the description
+- dmas: DMA specifiers for tx and rx dma. See the DMA client binding,
+Documentation/devicetree/bindings/dma/dma.txt.
+- dma-names: DMA request names should include "tx" and "rx" if present.
 
 Example: