refactor(rse): put MHU code in a dedicated file

To be able to use RSE comms without MHU, a first step is to disentangle
the rse_comms.c file with MHU code direct calls. This is done with the
creation of a new file rse_comms_mhu.c. New APIs are created to
initialize the mailbox, get max message size and send and receive data.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I75dda77e1886beaa6ced6f92c311617125918cfa
6 files changed