dt-bindings: pinctrl: Document the i.MX50 IOMUXC binding

AFAICS from the i.MX50 Reference Manual, the i.MX50 IOMUXC works the
same as the one in i.MX51, so I copied fsl,imx51-pinctrl.txt and changed
the text to imx50.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Cc: Dong Aisheng <dong.aisheng@linaro.org>
Cc: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>