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.com


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