SYSCON reboot mode driver

This driver gets 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 property:
- mask: bits mask of the bits in the register to store the reboot mode magic value,
  default set to 0xffffffff if missing.

The rest of the properties should follow the generic reboot-mode description
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-bootloader = <BOOT_FASTBOOT>;
			mode-loader = <BOOT_BL_DOWNLOAD>;
		};
	};
