ARM: dts: imx6q(dl): Move 'port' nodes under 'ports' for HDMI encoder

In preparation for the conversion of the DWC HDMI TX device tree
bindings to YAML, move the HDMI encoder's 'port' nodes under a 'ports'
node. The 'ports' node is optional in the OF graph implementation, but
YAML bindings require it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>