rpmsg: Support drivers without primary endpoint

Some types of rpmsg drivers does not have a primary endpoint to tie
their existence upon, but wishes to create and destroy endpoints
dynamically, e.g. based on user interactions.

Allow rpmsg drivers to omit a driver callback to signal this case and
make the probe path not create a primary endpoint in this case.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>