dt-bindings: dp-connector: add binding for DisplayPort connector

Add binding for DisplayPort connector. A few notes:

* Similar to hdmi-connector, it has hpd-gpios as an optional property,
  as the HPD could also be handled by, e.g., the DP bridge.

* dp-pwr-supply, which provides 3.3V on DP_PWR pin, is optional, as it
  is not strictly required: standard DP cables do not even have the pin
  connected.

* Connector type. Full size and mini connectors are identical except for
  the connector size and form, so I believe there is no functional need
  for this property. But similar to 'label' property, it might be used
  to present information about the connector to the userspace.

* No eDP. There's really no "eDP connector", as it's always a custom
  made connection between the DP and the DP panel, although the eDP spec
  does offer a few suggested pin setups. So possibly there is no need for
  edp-connector binding, but even if there is, I don't want to guess what
  it could look like, and could it be part of the dp-connector binding.

* No DP++. I'm not familiar with DP++. DP++ might need an i2c bus added
  to the bindings.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201130112919.241054-2-tomi.valkeinen@ti.com