imx-drm: add imx6 DT configuration for HDMI

Extracted from another patch by Fabio Estevam, this adds the DT
configuration for HDMI output on the IMX6 SoCs

Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>