spi: dt-bindings: fsl-dspi: add dmas and dma-names properties

Add dmas and dma-names properties because dspi support dma transfer.
Fix below warnings:
arch/arm64/boot/dts/freescale/fsl-ls1028a-kontron-sl28-var1.dtb: spi@2120000: Unevaluated properties are not allowed ('dma-names', 'dmas', 'little-endian' were unexpected)
        from schema $id: http://devicetree.org/schemas/spi/fsl,dspi.yaml#

Reviewed-by: Krzysztof Kozlowski <[email protected]>
Signed-off-by: Frank Li <[email protected]>
Reviewed-by: Vladimir Oltean <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Mark Brown <[email protected]>

Author: nxpfrankli

Documentation/devicetree/bindings/spi/fsl,dspi.yaml

@@ -48,6 +48,16 @@ properties:
     items:
       - const: dspi
 
+  dmas:
+    items:
+      - description: DMA controller phandle and request line for TX
+      - description: DMA controller phandle and request line for RX
+
+  dma-names:
+    items:
+      - const: tx
+      - const: rx
+
   spi-num-chipselects:
     $ref: /schemas/types.yaml#/definitions/uint32
     description: