dt-bindings: remoteproc: mediatek: Improve the rpmsg subnode definition

Improve the definition of the rpmsg subnode by assigning a distinct
node name and adding the definition source of node properties.

Signed-off-by: Tinghan Shen <tinghan.shen@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20230901080935.14571-2-tinghan.shen@mediatek.com


Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>