remoteproc: Properly deal with a detach request when attached

This patch introduces the capability to detach a remote processor
that has been attached to by the remoteproc core.  For that to happen
a rproc::ops::detach() 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/20210312162453.1234145-17-mathieu.poirier@linaro.org
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>