[media] dt-bindings: media/s5p-cec.txt: document needs-hpd property

Needed for boards that wire the CEC pin in such a way that it
is unavailable when the HPD is low.
Signed-off-by: NHans Verkuil <hans.verkuil@cisco.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/devicetree/bindings/media/s5p-cec.txt

@@ -17,6 +17,12 @@ Required properties:
   - samsung,syscon-phandle - phandle to the PMU system controller
   - hdmi-phandle - phandle to the HDMI controller
 
+Optional:
+  - needs-hpd : if present the CEC support is only available when the HPD
+    is high. Some boards only let the CEC pin through if the HPD is high, for
+    example if there is a level converter that uses the HPD to power up
+    or down.
+
 Example:
 
 hdmicec: cec@100B0000 {