stm32mp1: Add copro image support for M4 firmware

Implements copro image loading with FIT.
Once image is loaded with remoteproc,
the M4 coprocessor is automatically started.

Signed-off-by: Loic Pallardy <loic.pallardy@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>