dt-bindings: mailbox: qcom,apcs: Add separate node for clock-controller

dt-bindings: mailbox: qcom,apcs: Add separate node for clock-controller

APCS "global" is sort of a "miscellaneous" hardware block that combines
multiple registers inside the application processor subsystem. Two distinct
use cases are currently stuffed together in a single device tree node:

 - Mailbox: to communicate with other remoteprocs in the system.
 - Clock: for controlling the CPU frequency.

These two use cases have unavoidable circular dependencies: the mailbox is
needed as early as possible during boot to start controlling shared
resources like clocks and power domains, while the clock controller needs
one of these shared clocks as its parent. Currently, there is no way to
distinguish these two use cases for generic mechanisms like fw_devlink.

This is currently blocking conversion of the deprecated custom "qcom,ipc"
properties to the standard "mboxes", see e.g. commit d92e9ea2f0f9
("arm64: dts: qcom: msm8939: revert use of APCS mbox for RPM"):
  1. remoteproc &rpm needs mboxes = <&apcs1_mbox 8>;
  2. The clock controller inside &apcs1_mbox needs
     clocks = <&rpmcc RPM_SMD_XO_CLK_SRC>.
  3. &rpmcc is a child of remoteproc &rpm

The mailbox itself does not need any clocks and should probe early to
unblock the rest of the boot process. The "clocks" are only needed for the
separate clock controller. In Linux, these are already two separate drivers
that can probe independently.

Break up the circular dependency chain in the device tree by separating the
clock controller into a separate child node. Deprecate the old approach of
specifying the clock properties as part of the root node, but keep them for
backwards compatibility.

Signed-off-by: Stephan Gerhold <stephan.gerhold@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>