dt-bindings: mailbox: Add binding for SDX55 APCS

Add devicetree YAML binding for SDX55 APCS GCC block. The APCS block
acts as the mailbox controller and also provides a clock output and
takes 3 clock sources (pll, aux, ref) as input.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>