dt-bindings: power: reset: add document for NVMEM based reboot-mode

Add the device tree bindings document for the NVMEM based reboot-mode
driver.
Signed-off-by: Nandor Han <nandor.han@vaisala.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

Documentation/devicetree/bindings/power/reset/nvmem-reboot-mode.txt

+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>;
+	};