The address space allocated to the SPIFI for direct execution allows a maximum of 128 MB of SPI flash to be mapped into the CPU memory space. In practice, the usable space is limited to the size of the connected device. You can have a memory bigger than 128 MB but only 128 MB of that memory will be mapped into the CPU memory.
Regarding the flash driver, please take a look to the section 12.2.2 for more information about the SPIFI flash drivers, if the device you want to use is not supported by our supplied Flash Drivers, sources to generate these drivers are supplied in the Examples/Flashdrivers subdirectory within the MCUXpresso IDE installation directory. Users may thus add support for new SPIFI devices if needed.
Hope it helps!
Technical Support Engineer