remoteproc: Make client initialize ops in rproc_subdev

In preparation of adding the additional prepare and unprepare operations
make the client responsible for filling out the function pointers of the
rproc_subdev. This makes the arguments to rproc_add_subdev() more
manageable, in particular when some of the functions are left out.

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