[media] dt-bindings: add media/cec.txt

Document common HDMI CEC bindings. Add this to the MAINTAINERS file
as well.

Signed-off-by: Hans Verkuil <[e-mail]>
Acked-by: Benjamin Gaignard <[e-mail]>
Acked-by: Sylwester Nawrocki <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

Documentation/devicetree/bindings/media/cec.txt

···
+Common bindings for HDMI CEC adapters
+
+- hdmi-phandle: phandle to the HDMI controller.
+
+- 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.

MAINTAINERS

···
 F:	include/media/cec-notifier.h
 F:	include/uapi/linux/cec.h
 F:	include/uapi/linux/cec-funcs.h
+F:	Documentation/devicetree/bindings/media/cec.txt
 
 CELL BROADBAND ENGINE ARCHITECTURE
 M:	Arnd Bergmann <[e-mail]>