media: cec-gpio: add HDMI CEC GPIO driver

Add a simple HDMI CEC GPIO driver that sits on top of the cec-pin framework.

While I have heard of SoCs that use the GPIO pin for CEC (apparently an
early RockChip SoC used that), the main use-case of this driver is to
function as a debugging tool.

By connecting the CEC line to a GPIO pin on a Raspberry Pi 3 for example
it turns it into a CEC debugger and protocol analyzer.

With 'cec-ctl --monitor-pin' the CEC traffic can be analyzed.

But of course it can also be used with any hardware project where the
HDMI CEC line is hooked up to a pull-up gpio line.

In addition this has (optional) support for tracing HPD changes if the
HPD is connected to a GPIO.

Signed-off-by: Hans Verkuil <[email protected]>
Reviewed-by: Linus Walleij <[email protected]>
Signed-off-by: Mauro Carvalho Chehab <[email protected]>

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