image: add new "copro" image type

Define new image type for coprocessor images.
It is used in FIT to identify the files loaded
with remoteproc command (elf or bin).

Signed-off-by: Loic Pallardy <loic.pallardy@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>