nrf53_bsim: Get IPC shared memory buffer size from DT

Before we were defining the buffer in the runner context
which is simpler and less error prone, but it had a hardcoded
size decoupled from DT as it could not be based on DT
information.
Instead, let's allocate the buffer in the application core
image. This allows us to size it based on the device tree
configuration.
Note that this then requires the application core image
to be present during link time of the final executable
when the IPC subsystem is used.

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