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: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: <stable@vger.kernel.org>      # for v4.15
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>