configs: imx: imx8mm_beacon: Add config option for QSPI booting

The imx8mm_beacon SOM has a QSPI part attached to the FSPI controller.
Update the header and spl files to support booting from NOR flash and
add imx8mm_beacon_fspi_defconfig to support this configuration.

Signed-off-by: Adam Ford <aford173@gmail.com>
3 files changed