Documentation: fsl-quadspi: Add fsl, imx6ul-qspi compatible string

new compatible string: "fsl,imx6ul-qspi".
Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

Documentation/devicetree/bindings/mtd/fsl-quadspi.txt

@@ -2,7 +2,7 @@
 
 Required properties:
   - compatible : Should be "fsl,vf610-qspi", "fsl,imx6sx-qspi",
-		 "fsl,imx7d-qspi"
+		 "fsl,imx7d-qspi", "fsl,imx6ul-qspi"
   - reg : the first contains the register location and length,
           the second contains the memory mapping address and length
   - reg-names: Should contain the reg names "QuadSPI" and "QuadSPI-memory"