remoteproc: add vendor resources handling

In order to allow rproc backend to handle vendor resources such as in
OpenAMP, add a handle_rsc hook. This hook allow the rproc backends to
handle vendor resources as they like. The hook will be called only for
vendor resources and should return RSC_HANDLED on successful resource
handling, RSC_IGNORED if resource was ignored, or a negative value on
error.

Signed-off-by: Clement Leger <cleger@kalray.eu>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>