mailbox: qcom-apcs-ipc: enable APCS clock device for MSM8996

MSM8996 also has the clock-related part of the APCS mailbox device.
Follow the usual pattern and create a child device to handle these
clocks.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>