ARM: dts: imx: Add flexspi node to i.MX8MP

Add flexspi DT node matching Linux kernel as of commit
d7cd74466651e ("arm64: dts: imx8mp: Reorder flexspi clock-names entry")

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>