MAINTAINERS: Add file patterns for remoteproc device tree bindings

Submitters of device tree binding documentation may forget to CC
the subsystem maintainer if this is missing.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Ohad Ben-Cohen <ohad@wizery.com>
Cc: linux-remoteproc@vger.kernel.org
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>