boards: arm: mps2_an521: Enable TFM testing in CI
boards: arm: mps2_an521: Enable TFM testing in CI

The mps2_an521_nonsecure exists for TFM and is also utilized as a config
for multicore samples.  We can enable just the TFM tests with only_tags
and get a bit of additional coverage in QEMU for the TFM integration.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr