nrf5x_bsim: Add support for IPC/OpenAMP and split BT stack

For the 5340 simulated boards:
* Now that the HW models include the IPC and MUTEX peripherals
  we can enable them in DT.
* Also enable the DT mbox definition and allocate its shared memory
* Set the default kconfiguration for the HEAD, IPC, MBOX and split
  BT stack as in the equivalent real targets.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>