samples: rpmsg_service: use mps2_an521_remote target for remote core

When building and running the rpmsg_service sample on MPS2 AN521
use the mps2_an521_remote target to build the Zephyr image
for the remote core.

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