boards: arm: mps2_an521: enable testing on mps2_an521 by default

mps2_an521 is the default board for ARMv8-M architecture with
support for Security Extension, and CI should test building and
running samples and tests on this board by default. As the focus
is on kernel, userspace, and arm test suites, certain tags are
set to be ignored when testing on this platform.

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