Add support for shutdown and wake on gpio

[ulope]

The raspberry pi has a very useful feature in that it can be woken from the shutdown state by shorting gpio3 to ground. Additionally there’s a dtoverlay for soft shutdown on gpio.

See https://gist.github.com/lbussy/9e81cbcc617952f1250e353bd42e7775#tldr-version for example.

it would be great if the cb1 / btt pi could also support this.