mailbox: qcom: Add sdm660 hmss compatible

The Qualcomm SDM660 platform has a APCS HMSS GLOBAL block, add the
compatible for this.

Signed-off-by: Konrad Dybcio <konradybcio@gmail.com>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>