dt-bindings: mailbox: qcom-ipcc: Add SM6375 compatible

Add a compatible for SM6375.

Signed-off-by: Konrad Dybcio <konrad.dybcio@somainline.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>