boards: arm: mps2_an521: force secure firmware image by default

In order to increase code coverage, we force building a Secure
Firmware image by default (i.e. with option
CONFIG_TRUSTED_EXECUTION_SECURE set), when building for
mps2_an521 board. CONFIG_TRUSTED_EXECUTION_SECURE enables
compiling-in all TrustZone-related code in the tree, that is,
all ARM-specific code inside #ifdef CONFIG_ARM_SECURE_FIRMWARE.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>