modules: nxp_imx: Add HAS_MCUX_FLEXSPI

Adds a hidden config symbol HAS_MCUX_FLEXSPI selected by NXP SoCs when
the FlexSPI peripheral is present. It will be used as a dependency for a
new FlexSPI flash driver to prevent users from accidentally enabling the
driver on platforms that don't have the necessary hardware.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>