[IMX6][PWR] add poweroff and reboot system with kill power GPIO

	this feature allow the cpu to turn off the power of the board on poweroff using a GPIO;
	the kill power GPIO have to be specified in the device tree with a power_signal node:

	power_signal: power_signal {
		power-gpio = <&gpio2 4 0>;
	};

	modified:   arch/arm/include/asm/mach/arch.h
	modified:   arch/arm/include/asm/system_misc.h
	modified:   arch/arm/kernel/reboot.c
	modified:   arch/arm/kernel/setup.c
	modified:   arch/arm/mach-imx/mach-imx6q.c