media: dt-bindings/media/cec-gpio.txt: mention the CEC/HPD max voltages

Mention the maximum voltages of the CEC and HPD lines. Since in the example
these lines are connected to a Raspberry Pi and the Rpi GPIO lines are 3.3V
it is a good idea to warn against directly connecting the HPD to the Raspberry
Pi's GPIO line.
Signed-off-by: NHans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Cc: <stable@vger.kernel.org>      # for v4.15
Signed-off-by: NHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>

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

@@ -4,6 +4,10 @@ The HDMI CEC GPIO module supports CEC implementations where the CEC line
 is hooked up to a pull-up GPIO line and - optionally - the HPD line is
 hooked up to another GPIO line.
 
+Please note: the maximum voltage for the CEC line is 3.63V, for the HPD
+line it is 5.3V. So you may need some sort of level conversion circuitry
+when connecting them to a GPIO line.
+
 Required properties:
   - compatible: value must be "cec-gpio".
   - cec-gpios: gpio that the CEC line is connected to. The line should be
@@ -21,7 +25,7 @@ the following property is optional:
 
 Example for the Raspberry Pi 3 where the CEC line is connected to
 pin 26 aka BCM7 aka CE1 on the GPIO pin header and the HPD line is
-connected to pin 11 aka BCM17:
+connected to pin 11 aka BCM17 (some level shifter is needed for this!):
 
 #include <dt-bindings/gpio/gpio.h>