ARM: dts: imx28-evk: Run I2C0 at 400kHz

It is safe to operate I2C0 at 400kHz as SGTL5000 supports this frequency.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>