[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: Hans Verkuil <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

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

···
   - 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 {