tty: add rpmsg driver

This driver exposes a standard TTY interface on top of the rpmsg
framework through a rpmsg service.

This driver supports multi-instances, offering a /dev/ttyRPMSGx entry
per rpmsg endpoint.

Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Link: https://lore.kernel.org/r/20211015094701.5732-3-arnaud.pouliquen@foss.st.com


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