ARM: dts: Set i2c7 clock at 400kHz for exynos based Peach boards

The downstream ChromeOS 3.8 kernel sets the clock frequency
for the I2C bus 7 at 400kHz. Do the same change in mainline.

Suggested-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>