SYSCON reboot mode driver

This driver get reboot mode magic value form reboot-mode driver
and stores it in a SYSCON mapped register. Then the bootloader
can read it and take different action according to the magic
value stored.

This DT node should be represented as a sub-node of a "syscon", "simple-mfd"
node.

Required properties:
- compatible: should be "syscon-reboot-mode"
- offset: offset in the register map for the storage register (in bytes)

Optional properity:
- mask: the mask bits of the mode magic value, default set to 0xffffffff if missing.

The rest of the properties should follow the generic reboot-mode discription
found in reboot-mode.txt

Example:
	pmu: pmu@20004000 {
		compatible = "rockchip,rk3066-pmu", "syscon", "simple-mfd";
		reg = <0x20004000 0x100>;

		reboot-mode {
			compatible = "syscon-reboot-mode";
			offset = <0x40>;
			mode-normal = <BOOT_NORMAL>;
			mode-recovery = <BOOT_RECOVERY>;
			mode-fastboot = <BOOT_FASTBOOT>;
			mode-loader = <BOOT_LOADER>;
		};
	};
