dt-bindings: spi: add compatible entry for imx8mp in FlexSPI controller

add compatible entry "nxp,imx8mp-fspi" in NXP FlexSPI controller
Signed-off-by: NHeiko Schocher <hs@denx.de>
Link: https://lore.kernel.org/r/20210316075928.1763460-3-hs@denx.deSigned-off-by: NMark 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,imx8mp-fspi"
 			    "nxp,imx8dxl-fspi"
 
   - reg :        First contains the register location and length,