remoteproc: Make start and stop in subdev optional

Some subdevices, such as glink ssr only care about the stop operation,
so make the operations optional to reduce client code.
Tested-by: NFabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: NBjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by Alex Elder <elder@linaro.org>
Signed-off-by: NBjorn Andersson <bjorn.andersson@linaro.org>

drivers/remoteproc/remoteproc_core.c

@@ -780,16 +780,20 @@ static int rproc_start_subdevices(struct rproc *rproc)
 	int ret;
 
 	list_for_each_entry(subdev, &rproc->subdevs, node) {
+		if (subdev->start) {
 			ret = subdev->start(subdev);
 			if (ret)
 				goto unroll_registration;
 		}
+	}
 
 	return 0;
 
 unroll_registration:
-	list_for_each_entry_continue_reverse(subdev, &rproc->subdevs, node)
+	list_for_each_entry_continue_reverse(subdev, &rproc->subdevs, node) {
+		if (subdev->stop)
 			subdev->stop(subdev, true);
+	}
 
 	return ret;
 }
@@ -798,8 +802,10 @@ static void rproc_stop_subdevices(struct rproc *rproc, bool crashed)
 {
 	struct rproc_subdev *subdev;
 
-	list_for_each_entry_reverse(subdev, &rproc->subdevs, node)
+	list_for_each_entry_reverse(subdev, &rproc->subdevs, node) {
+		if (subdev->stop)
 			subdev->stop(subdev, crashed);
+	}
 }
 
 /**