remoteproc: Properly deal with a stop request when attached

Allow a remote processor that was started by another entity to be
switched off by the remoteproc core.  For that to happen a
rproc::ops::stop() operation needs to be available.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Reviewed-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Link: https://lore.kernel.org/r/20210312162453.1234145-16-mathieu.poirier@linaro.org
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>