arm64: dts: qcom: msm8976: add compatible fallback to mailbox

MSM8976 mailbox is compatible with MSM8994.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230322174148.810938-5-krzysztof.kozlowski@linaro.org