mailbox: qcom: Add ipq6018 apcs compatible

The Qualcomm ipq6018 has apcs block, add compatible for the same. Also,
the ipq6018 apcs provides a clock functionality similar to msm8916 but
the clock driver is different.

Create a child device based on the apcs compatible for the clock
controller functionality.

Signed-off-by: Sivaprakash Murugesan <sivaprak@codeaurora.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>