remoteproc: Remove vdev_to_rvdev and vdev_to_rproc from remoteproc API

These both functions are only used by the remoteproc_virtio.
There is no reason to expose them in the API.
Move the functions in remoteproc_virtio.c

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20211001101234.4247-4-arnaud.pouliquen@foss.st.com