spi: spi-nxp-fspi: Add imx8dxl support

Add support for nxp,imx8dxl-fspi, do the bindings document.
Signed-off-by: Kuldeep Singh <kuldeep.singh@nxp.com>
Link: https://lore.kernel.org/r/20210302124936.1972546-3-kuldeep.singh@nxp.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-nxp-fspi.txt

@@ -4,6 +4,7 @@ Required properties:
   - compatible : Should be "nxp,lx2160a-fspi"
 			    "nxp,imx8qxp-fspi"
 			    "nxp,imx8mm-fspi"
+			    "nxp,imx8dxl-fspi"
 
   - reg :        First contains the register location and length,
                  Second contains the memory mapping address and length