mailbox: qcom: Create APCS child device for clock controller

There is a clock controller functionality provided by the APCS hardware
block of msm8916 devices. The device-tree would represent an APCS node
with both mailbox and clock provider properties.
Create a platform child device for the clock controller functionality so
the driver can probe and use APCS as parent.

Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>
Acked-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>