dt-bindings: mailbox: qcom: Document the APCS clock binding

Update the binding documentation for APCS to mention that the APCS
hardware block also expose a clock controller functionality.

The APCS clock controller is a mux and half-integer divider. It has the
main CPU PLL as an input and provides the clock for the application CPU.

Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Bjorn Andersson <bjorn.andersson@linaro.org>