mailbox: qcom: Add msm8998 hmss compatible

The Qualcomm MSM8998 platform has a APCS HMSS GLOBAL block, add the
compatible for this.

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