dt-bindings: display: allow port and ports in panel-lvds

Both port and ports names may be used in a panel-lvds binding
  port - for a single port
  ports - if there is more than one port in sub-nodes

Fixes the following warning:
advantech,idk-2121wr.example.dt.yaml: panel-lvds: 'port' is a required property

advantech,idk-2121wr.yaml needs several ports, so uses a ports node.

v2:
  - Use oneOf - makes the logic more obvious (Rob)
  - Added Fixes tag
  - Added port: true, ports:true

v3:
  - Indent port/ports in required two spaces (Rob)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Rob Herring <robh@kernel.org>
Fixes: 8efef33e ("dt-bindings: display: Add idk-2121wr binding")
Cc: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Cc: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20200412132139.11418-2-sam@ravnborg.org