ipc: Fix icmsg deregister functionality

This change fixes the deregister functionality for
icmsg backend. There are two changes:

Memory access layer was not initialized properly
when the endpoint was registered again. This lead to
memory fault when attepmt to write to the buffer was
done.

Mbox driver was initialized before memory access layer
was initialized. This could lead to race condition
where immediately after the mbox driver was initialized,
the receiving work is scheduled due to pending interrupt.
In that case an attempt to read from garbage address will
occur.

Signed-off-by: Emil Obalski <Emil.Obalski@nordicsemi.no>