gpio: mxc: Add clocks optional binding for imx gpio

Some i.MX SoCs have GPIO clock gate in CCM, accessing
GPIO registers needs to enable GPIO clock gate first,
i.MX GPIO driver will enable clock gate if there is
clock property in GPIO node of dtb, add optional property
to i.MX GPIO binding doc.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>