drivers: flash: flash_mcux_flexspi_nor: reduce LUT usage

FLEXSPI nor driver uses several LUTs for "scratch" commands during the
SFDP probe phase that are not needed once the flash is configured. Set a
second "end" marker we can use to configure the FLEXSPI MEMC driver, so
that we can indicate the true number of LUTs needed by this driver when
init is completed.

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