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 <kumar.gala@linaro.org>