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.com

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200507143354.v5.2.I1976736b400a3b30e46efa47782248b86b3bc627@changeid