GPIO Reboot Mode Configuration

Required Properties:
- compatible: must be "reboot-mode-gpio".
- gpios: list of gpios that are used to calculate the reboot-mode magic value.
	 Every gpio represents a bit in the magic value in the same order
	 as defined in device tree.
- modes: list of properties that define the modes and associated unique ids.

Optional Properties:
- u-boot,env-variable: used to save the reboot mode (default: reboot-mode).

Example:
	reboot-mode {
		compatible = "reboot-mode-gpio";
		gpios = <&gpio1 2 GPIO_ACTIVE_LOW>, <&gpio2 6 GPIO_ACTIVE_HIGH>;
		u-boot,env-variable = "bootstatus";
		mode-test = <0x00000001>;
		mode-download = <0x00000002>;
	};
