boards: mps2_an521: add an mps2_an521_remote target

Add a new target for MPS2 AN521, called mps2_an521_remote.
This target is used in dual image MPS2 AN521 builds, for
the 'remote' target. It is indentical to mps2_an521_nonsecure,
except for the fact that it does not enable TrustZone-M and
it does not set the security domain to Non-Secure.

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