i2c: imx: Include the right GPIO header

<linux/of_gpio.h> is not used in this file, by
<linux/gpio/consumer.h> is.

Someone is just lucky with their implicit includes.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>