boards: posix: nrf52_sim: model DSB as no-op in cmsis.h

Provide a very simple model of the DSB ARM
instruction as no-operation. The implementation
is provided in a cmsis.h header file added
in the nrf52_bsim board directory.

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