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>