dt-bindings: HID: i2c-hid: Add "panel" property to i2c-hid backed touchscreens

As talked about in the patch ("drm/panel: Add a way for other devices
to follow panel state"), touchscreens that are connected to panels are
generally expected to be power sequenced together with the panel
they're attached to. Today, nothing provides information allowing you
to find out that a touchscreen is connected to a panel. Let's add a
phandle for this.

The proerty is added to the generic touchscreen bindings and then
enabled in the bindings for the i2c-hid backed devices. This can and
should be added for other touchscreens in the future, but for now
let's start small.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Benjamin Tissoires <bentiss@kernel.org>
Acked-by: Benjamin Tissoires <bentiss@kernel.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230727101636.v4.1.Id68e30343bb1e11470582a9078b086176cfec46b@changeid

Documentation/devicetree/bindings/input/touchscreen/touchscreen.yaml

@@ -10,6 +10,13 @@ maintainers:
   - Dmitry Torokhov <dmitry.torokhov@gmail.com>
 
 properties:
+  panel:
+    description: If this touchscreen is integrally connected to a panel, this
+      is a reference to that panel. The presence of this reference indicates
+      that the touchscreen should be power sequenced together with the panel
+      and that they may share power and/or reset signals.
+    $ref: /schemas/types.yaml#/definitions/phandle
+
   touchscreen-min-x:
     description: minimum x coordinate reported
     $ref: /schemas/types.yaml#/definitions/uint32