MAINTAINERS: Add rpmsg tty driver maintainer

Adding myself as rpmsg tty maintainer and also adding remoteproc
mailing list to inform about changes in the driver.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Link: https://lore.kernel.org/r/20211102123817.19874-1-arnaud.pouliquen@foss.st.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>