dt-bindings: mailbox: qcom-ipcc: Add compatible for QDU1000/QRU1000

Document the compatible for the QDU1000 mailbox.

Signed-off-by: Melody Olvera <quic_molvera@quicinc.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>