dt-bindings: display: simple: List hpd properties in panel-simple

dt-bindings: display: simple: List hpd properties in panel-simple

The HPD (Hot Plug Detect) signal is present in many (probably even
"most") eDP panels. For eDP, this signal isn't actually used for
detecting hot-plugs of the panel but is more akin to a "panel ready"
signal. After you provide power to the panel, panel timing diagrams
typically say that you should wait for HPD to be asserted (or wait a
fixed amount of time) before talking to the panel.

The panel-simple bindings describes many eDP panels and many of these
panels provide the HPD signal. We should add the HPD-related
properties to the panel-simple bindings. The HPD properties are
actually defined in panel-common.yaml, so adding them here just
documents that they are OK for panels handled by the panel-simple
bindings.

NOTE: whether or not we'd include HPD properties in the panel node is
more a property of the board design than the panel itself. For most
boards using these eDP panels everything "magically" works without
specifying any HPD properties and that's been why we haven't needed to
allow the HPD properties earlier. On these boards the HPD signal goes
directly to a dedicated "HPD" input to the eDP controller and this
connection doesn't need to be described in the device tree. The only
time the HPD properties are needed in the device tree are if HPD is
hooked up to a GPIO or if HPD is normally on the panel but isn't used
on a given board. That means that if we don't allow the HPD properties
in panel-simple then one could argue that we've got to boot all eDP
panels (or at least all those that someone could conceivably put on a
system where HPD goes to a GPIO or isn't hooked up) from panel-simple.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210611101711.v10.1.Ieb731d23680db4700cc41fe51ccc73ba0b785fb7@changeid