add compatible entry "nxp,imx8mp-fspi" in NXP FlexSPI controller

Signed-off-by: Heiko Schocher <hs@denx.de>
Link: https://lore.kernel.org/r/20210316075928.1763460-3-hs@denx.de


Signed-off-by: Mark Brown <broonie@kernel.org>