dt-bindings: display: convert panel-dpi to DT schema

With panel-timing converted, now convert the single
remaining .txt user in panel/ of panel-timing to DT schema.

v2:
  - Drop Thierry as maintainer, as this is not a general panel binding
    and I have no acks.
  - Drop requirement for a panel- specific binding - "panel-dpi" is enough
  - Updated example

v3:
  - added yaml document terminator "..."
  - always require a specific binding - panel-dpi (based on feedback from Rob)
  - use "power-supply" for the supply property, and made it mandatory
    "power-supply" is the standard property for panels

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Maxime Ripard <mripard@kernel.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200216181513.28109-4-sam@ravnborg.org