Documentation: fsl-quadspi: Add fsl,ls1021-qspi compatible string

new compatible string: "fsl,ls1021-qspi".
Signed-off-by: Yuan Yao <yao.yuan@freescale.com>
Signed-off-by: Rob Herring <robh@kernel.org>

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

@@ -2,7 +2,8 @@
 
 Required properties:
   - compatible : Should be "fsl,vf610-qspi", "fsl,imx6sx-qspi",
-		 "fsl,imx7d-qspi", "fsl,imx6ul-qspi"
+		 "fsl,imx7d-qspi", "fsl,imx6ul-qspi",
+		 "fsl,ls1021-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"