ARM: dts: imx6dl: add alias for I2C4 bus

On registration I2C bus drivers attemp to get ids from device tree
aliases, add a missing alias for I2C4 found on iMX6 DualLite/Solo.

Signed-off-by: Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>