boards: imx_rt: Don't default FLASH_MCUX_FLEXSPI_XIP to enabled

Do not default FLASH_MCUX_FLEXSPI_XIP to enabled when code is not
located in flash, this will cause issues if code is executing from ITCM,
as the zephyr_code_relocate macro will relocate the flash driver code
into itcm, and overwrite the zephyr image.

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