mx7dsabresd_secure_defconfig: Use CONFIG_ARMV7_BOOT_SEC_DEFAULT

There is no need for introducing MX7_SEC, as there is the
CONFIG_ARMV7_BOOT_SEC_DEFAULT option for this purpose.

Switch to CONFIG_ARMV7_BOOT_SEC_DEFAULT and get rid of
MX7_SEC.

Tested by booting a 4.1.15 NXP kernel with mx7dsabresd_secure_defconfig
target.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Stefan Agner <stefan.agner@toradex.com>