dt-bindings: display: Add hpd-gpios to panel-common bindings

In the cases where there is no connector in a system there's no great
place to put "hpd-gpios".  As per discussion [1] the best place to put
it is in the panel.  Add this to the device tree bindings.

[1] https://lore.kernel.org/r/20200417180819.GE5861@pendragon.ideasonboard.comSigned-off-by: NDouglas Anderson <dianders@chromium.org>
Reviewed-by: NStephen Boyd <swboyd@chromium.org>
Reviewed-by: NLinus Walleij <linus.walleij@linaro.org>
Reviewed-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200507143354.v5.2.I1976736b400a3b30e46efa47782248b86b3bc627@changeid

Documentation/devicetree/bindings/display/panel/panel-common.yaml

@@ -96,6 +96,12 @@ properties:
       (hot plug detect) signal, but the signal isn't hooked up so we should
       hardcode the max delay from the panel spec when powering up the panel.
 
+  hpd-gpios:
+    maxItems: 1
+    description:
+      If Hot Plug Detect (HPD) is connected to a GPIO in the system rather
+      than a dedicated HPD pin the pin can be specified here.
+
   # Control I/Os
 
   # Many display panels can be controlled through pins driven by GPIOs. The nature