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: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by Alex Elder <elder@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>