ipc: backend: rpmsg: Add support for POSIX arch

To enable runninng and testing this backend on the POSIX
architecture, we need to allow defining the shared memory
buffer as a compiler provided symbol, instead of a hard
address provided by the DTS.
For this case we refer to a symbol a POSIX arch implementation
(typically the board) needs to provide.
It is the responsability of that implementation to ensure
that symbol exists and points to a memory buffer of the
DT defined size.

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