ARM: dts: imx6sx-udoo-neo: Add HDMI support

imx6sx-udoo-neo has a TDA19988 HDMI transmitter connected to the
parallel eLCDIF interface.

Add support for it.

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