tty: rpmsg: 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: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20211025135148.53944-4-andriy.shevchenko@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>