ARM: dts: imx23: Add i2c support

The imx23-olinuxino board has an i2c interface exposed on UEXT connector.
This patch provides the generic devicetree infrastructure for a follow-up
patch to add support to the actual board files.
Tested all three possible pin groups with MOD-LCD1x9 from Olimex.

This patch is based on work by Fadil Berisha with his permission. However
all bugs are mine.

Signed-off-by: Harald Geyer <harald@ccbib.org>
Acked-by: Stefan Wahren <stefan.wahren@i2se.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>