MAINTAINERS: Add co-maintainer for remoteproc/RPMSG subsystems

After discussing with Bjorn, stepping forward to help with the
maintenance of the remoteproc and RPMSG subsystems.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20210104171618.2702461-1-mathieu.poirier@linaro.org


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>