drivers: memc: memc_mcux_flexspi: support initializing FLEXSPI when XIP

Add support for initializing the FLEXSPI when using a flash attached to
the FLEXSPI for XIP. This option is guarded behind a Kconfig, as
enabling it is dangerous and requires special care be taken by the user
to ensure that the configuration of pins and FLEXSPI settings will not
break support for reading the attached flash, as this will break XIP
support.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>