dt-bindings: remoteproc: Add Xilinx RPU subsystem bindings

Xilinx ZynqMP platform has dual-core ARM Cortex R5 Realtime Processing
Unit(RPU) subsystem. This patch adds dt-bindings for RPU subsystem
(cluster).

Signed-off-by: Tanmay Shah <tanmay.shah@amd.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20221114233940.2096237-2-tanmay.shah@amd.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>