ARM: dts: imx6sx-sdb: add i2c3 node

This patch adds node i2c bus 3 to get the appropriate userland device
file. So for prototyping it's possible to experiment within userland.

Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>