pinctrl: add pinctrl driver for imx6dl

The imx6dl is a derivative of imx6q with very limited difference.  These
two SoCs are so compatible that they can be handled as one platform in
software.  That said, we will not have target SOC_IMX6DL but just
reusing SOC_IMX6Q.   That's why the pinctrl-imx6dl driver is added here
with symbol PINCTRL_IMX6Q controlling the build of it.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>