pinctrl: imx50: add pinctrl support code for the IMX50 SoC

Add code to support the specific pin arrangements of the Freescale IMX50 SoC.

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