mailbox: qcom: Add msm8994 apcs compatible

MSM8994 has an APCS block similar to 8916, but
with a different clock driver due to the former
one having 2 clusters.

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