mx6saberesd_spl: Correct falcon mode addition

When falcon mode support was added, it was right around when SPL_OS_BOOT
was migrated to Kconfig.  So first we must move the enablement to the
defconfig file.  Next, it turned off EXT support rather than add the
information to allow for falcon mode from EXT.  Add this information so
that the board compiles after 5d28b930.

Fixes: d96796ca ("mx6sabresd: Add Falcon mode support")
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Tom Rini <trini@konsulko.com>