rpmsg: Make modalias work for DeviceTree based devices

When rpmsg devices are expected to be matched based on their compatible
the modalias should reflect this, so that module autoloading has a
chance to match and load the appropriate module.

Tested-by: Rob Clark <robdclark@gmail.com>
Reported-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>