mx7dsabresd: Fix the boot of a NXP kernel

mx7dsabresd: Fix the boot of a NXP kernel

Booting a NXP kernel with mainline U-boot leads to the following kernel
crash:

caam: probe of 30900000.caam failed with error -11
Unable to handle kernel NULL pointer dereference at virtual address 00000004
pgd = 80004000
[00000004] *pgd=00000000
Internal error: Oops: 805 [#1] PREEMPT SMP ARM

This happens because NXP kernel expects MX7 to boot in secure mode,
so introduce mx7dsabresd_secure_defconfig that selects CONFIG_MX7_SEC
and allows booting a NXP provided kernel successfully.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>