mailbox: qcom-apcs-ipc: Assign OF node to clock controller child device

Currently, the child device for the clock controller inside the APCS block
is created without any OF node assigned, so the drivers need to rely on the
parent device for obtaining any resources.

Add support for defining the clock controller inside a "clock-controller"
subnode to break up circular dependencies between the mailbox and required
parent clocks of the clock controller. For backwards compatibility, if the
subnode is not defined, reuse the OF node from the parent device.

Signed-off-by: Stephan Gerhold <stephan.gerhold@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>