dt-bindings: spi: spi-cadence: update DT binding docs to support cix sky1 SoC

- Add new compatible strings to the DT binding documents to support
cix sky1 SoC.

Signed-off-by: Jun Guo <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/spi/spi-cadence.yaml

···
           - enum:
               - xlnx,zynqmp-spi-r1p6
               - xlnx,versal-net-spi-r1p6
+              - cix,sky1-spi-r1p6
           - const: cdns,spi-r1p6
 
   reg: