dt-bindings: exynos-dp: update ports node binding for panel

This patch updates a ports node binding for panel.

With this, dp node can have a ports node which describes
a remote endpoint node that can be connected to panel or bridge
node.

Changelog v2:
- remove unnecessary properties and numbering.
- update description about eDP device.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Rob Herring <robh@kernel.org>