dt-bindings: remoteproc: Add Renesas R-Car

Renesas R-Car SoCs may contain a Realtime processor.
This patch adds bindings for this remote processor.

Signed-off-by: Julien Massot <julien.massot@iot.bzh>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20211207165829.195537-2-julien.massot@iot.bzh
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>