Doc/DT: hdmi-connector: add HPD GPIO documentation

Add binding documentation for HDMI connector's HPD GPIO.
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@ti.com>
Cc: devicetree@vger.kernel.org

Documentation/devicetree/bindings/video/hdmi-connector.txt

@@ -7,6 +7,7 @@ Required properties:
 
 Optional properties:
 - label: a symbolic name for the connector
+- hpd-gpios: HPD GPIO number
 
 Required nodes:
 - Video port for HDMI input