remoteproc: st: add virtio communication support

This patch provides virtio communication support based on mailbox
for ST co-processors.

Signed-off-by: Loic Pallardy <loic.pallardy@st.com>
Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>