spi: nxp_fspi: Add i.MX8MP compatible string

The i.MX8M Mini and i.MX8M Plus flexspi IPs are compatible with one
another, however the linux kernel DT uses separate compatible string
for each SoC. Add the missing i.MX8MP compatible into this driver.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@denx.de>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>