soc: nxp: MCXNx4x: enable booting from QSPI flash

The ROM bootloader has the option to boot from external QSPI flash on
the FlexSPI instead of internal flash.  Adds
CONFIG_NXP_FLEXSPI_BOOT_HEADER to include the FlexSPI boot ROM header
in the image.

Signed-off-by: Derek Snell <derek.snell@nxp.com>