ARM: dts: imx6sx-udoo-neo: add bluetooth config to uart3

This updates the WL1831 bluetooth to use the serial serdev driver
interface.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>