dt-bindings: gpio: vf610: add optional clocks property

On some SoCs(e.g. MX7ULP), GPIO clock is gatable and maybe
disabled by default. Users have to make sure it's enabled before
being able to access controller registers, otherwise an external
abort error may occur. Let's add the optional clocks property to
handle this case.

For ULP GPIO clock, it includes two separate clocks: one is for
GPIO controller Input/Output function clock while another is
GPIO port control clock for interrupt function.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Stefan Agner <stefan@agner.ch>
Cc: linux-gpio@vger.kernel.org
Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>