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

MAINTAINERS

@@ -15205,6 +15205,7 @@ F:	fs/reiserfs/
 REMOTE PROCESSOR (REMOTEPROC) SUBSYSTEM
 M:	Ohad Ben-Cohen <ohad@wizery.com>
 M:	Bjorn Andersson <bjorn.andersson@linaro.org>
+M:	Mathieu Poirier <mathieu.poirier@linaro.org>
 L:	linux-remoteproc@vger.kernel.org
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/andersson/remoteproc.git rproc-next
@@ -15218,6 +15219,7 @@ F:	include/linux/remoteproc/
 REMOTE PROCESSOR MESSAGING (RPMSG) SUBSYSTEM
 M:	Ohad Ben-Cohen <ohad@wizery.com>
 M:	Bjorn Andersson <bjorn.andersson@linaro.org>
+M:	Mathieu Poirier <mathieu.poirier@linaro.org>
 L:	linux-remoteproc@vger.kernel.org
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/andersson/remoteproc.git rpmsg-next