ARM: dts: imx27 iomux device node

This patch adds a iomux node for imx27 pinctrl driver. The gpio
registers are embedded in the iomux memory area. So this patch moves
them into the iomux node for a better hardware description.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>