gpio: aspeed: dt: Add optional clocks property

We need a reference to the HPLL to calculate debounce cycles. If the
clocks property is not supplied in the GPIO node then the consumer
should deny any debounce requests.
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/gpio/gpio-aspeed.txt

@@ -18,6 +18,7 @@ Required properties:
 Optional properties:
 
 - interrupt-parent      : The parent interrupt controller, optional if inherited
+- clocks                : A phandle to the HPLL clock node for debounce timings
 
 The gpio and interrupt properties are further described in their respective
 bindings documentation: