logind: add standard gpio power button support

logind: add standard gpio power button support

Many boards like hisilicon D02 board use standard gpio key to power down system.
A description of gpio-key in dts shown below,

gpio_keys {
	compatible = "gpio-keys";
	#address-cells = <1>;
	#size-cells = <0>;

	pwrbutton {
		label = "Power Button";
		gpios = <&porta 8 1>;
		linux,code = <116>;  // KEY_POWER, used by SC System Power Down
	};
};

-bash-4.3# udevadm info -a /dev/input/event3

Udevadm info starts with the device specified by the devpath and then
walks up the chain of parent devices. It prints for every device
found, all possible attributes in the udev rules key format.
A rule to match, can be composed by the attributes of the device
and the attributes from one single parent device.

  looking at device '/devices/platform/gpio_keys/input/input3/event3':
    KERNEL=="event3"
    SUBSYSTEM=="input"
    DRIVER==""

  looking at parent device '/devices/platform/gpio_keys/input/input3':
    KERNELS=="input3"
    SUBSYSTEMS=="input"
    DRIVERS==""
    ATTRS{name}=="gpio_keys"
    ATTRS{phys}=="gpio-keys/input0"
    ATTRS{uniq}==""
    ATTRS{properties}=="0"

  looking at parent device '/devices/platform/gpio_keys':
    KERNELS=="gpio_keys"
    SUBSYSTEMS=="platform"
    DRIVERS=="gpio-keys"
    ATTRS{keys}=="116"
    ATTRS{switches}==""
    ATTRS{driver_override}=="(null)"
    ATTRS{disabled_keys}==""
    ATTRS{disabled_switches}==""

  looking at parent device '/devices/platform':
    KERNELS=="platform"
    SUBSYSTEMS==""
    DRIVERS==""