spi: spi-nxp-fspi: Add support for imx8mm, imx8qxp

Add support for nxp,imx8qxp-fspi and nxp,imx8mm-fspi do the bindings
document.
Signed-off-by: Adam Ford <aford173@gmail.com>

Link: https://lore.kernel.org/r/20200126140913.2139260-4-aford173@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

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

@@ -2,6 +2,9 @@
 
 Required properties:
   - compatible : Should be "nxp,lx2160a-fspi"
+			    "nxp,imx8qxp-fspi"
+			    "nxp,imx8mm-fspi"
+
   - reg :        First contains the register location and length,
                  Second contains the memory mapping address and length
   - reg-names :  Should contain the resource reg names: