remoteproc: Rename subdev functions to start/stop

"start" and "stop" are more suitable names for how these two operations
are used, and they fit better with the upcoming introduction of two
additional operations in the struct.

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