rpmsg: char: Add pr_fmt() to prefix messages

Make all messages to be prefixed in a unified way.
Add pr_fmt() to achieve this.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Link: https://lore.kernel.org/r/20211108135945.3364-1-arnaud.pouliquen@foss.st.com


Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>