dt-bindings: mailbox: qcom: Add clock-name optional property

When the APCS clock is registered (platform dependent), it retrieves
its parent names from hardcoded values in the driver.

The following commit allows the DT node to provide such clock names to
the platform data based clock driver therefore avoiding having to
explicitly embed those names in the clock driver source code.

Co-developed-by: Niklas Cassel <niklas.cassel@linaro.org>
Signed-off-by: Niklas Cassel <niklas.cassel@linaro.org>
Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>