NVMEM reboot mode driver

This driver gets reboot mode magic value from reboot-mode driver
and stores it in a NVMEM cell named "reboot-mode". Then the bootloader
can read it and take different action according to the magic
value stored.

Required properties:
- compatible: should be "nvmem-reboot-mode".
- nvmem-cells: A phandle to the reboot mode provided by a nvmem device.
- nvmem-cell-names: Should be "reboot-mode".

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

Example:
	reboot-mode {
		compatible = "nvmem-reboot-mode";
		nvmem-cells = <&reboot_mode>;
		nvmem-cell-names = "reboot-mode";

		mode-normal     = <0xAAAA5501>;
		mode-bootloader = <0xBBBB5500>;
		mode-recovery   = <0xCCCC5502>;
		mode-test       = <0xDDDD5503>;
	};