Documentation: fsl-quadspi: Add an entry for the imx6sx compatible string

"fsl,imx6sx-qspi" is also a valid compatible string, so add an entry for it.
Signed-off-by: NFabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: NBrian Norris <computersforpeace@gmail.com>

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

 * Freescale Quad Serial Peripheral Interface(QuadSPI)
 
 Required properties:
-  - compatible : Should be "fsl,vf610-qspi"
+  - compatible : Should be "fsl,vf610-qspi" or "fsl,imx6sx-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"