Driver a GPIO line that can be used to turn the power off.

The driver supports both level triggered and edge triggered power off.
At driver load time, the driver will request the given gpio line and
install a handler to power off the system. If the optional properties
'input' is not found, the GPIO line will be driven in the inactive
state. Otherwise its configured as an input.

When the power-off handler is called, the gpio is configured as an
output, and drive active, so triggering a level triggered power off
condition. This will also cause an inactive->active edge condition, so
triggering positive edge triggered power off. After a delay of 100ms,
the GPIO is set to inactive, thus causing an active->inactive edge,
triggering negative edge triggered power off. After another 100ms
delay the GPIO is driver active again. If the power is still on and
the CPU still running after a 3000ms delay, a WARN_ON(1) is emitted.

Required properties:
- compatible : should be "gpio-poweroff".
- gpios : The GPIO to set high/low, see "gpios property" in
  Documentation/devicetree/bindings/gpio/gpio.txt. If the pin should be
  low to power down the board set it to "Active Low", otherwise set
  gpio to "Active High".

Optional properties:
- input : Initially configure the GPIO line as an input. Only reconfigure
  it to an output when the power-off handler is called. If this optional
  property is not specified, the GPIO is initialized as an output in its
  inactive state.
- timeout-ms: Time to wait before asserting a WARN_ON(1). If nothing is
              specified, 3000 ms is used.

Examples:

gpio-poweroff {
	compatible = "gpio-poweroff";
	gpios = <&gpio 4 0>;
	timeout-ms = <3000>;
};
