pinctrl: imx: Catch no fsl,pins property

Instead of crashing the kernel print an error message when
the fsl,pins property is missing.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>