dt-bindings: power: reset: add document for reboot-mode driver

Generic reboot mode core map driver

This driver get reboot mode arguments and call the write

interface to store the magic value in special register

or ram. Then the bootloader can read it and take different

action according to the argument stored.

All mode properties are vendor specific, it is a indication to tell

the bootloader what to do when the system reboots, and should be named

as mode-xxx = <magic> (xxx is mode name, magic should be a none-zero value).

For example modes common on Android platform:

- mode-normal: Normal reboot mode, system reboot with command "reboot".

- mode-recovery: Android Recovery mode, it is a mode to format the device or update a new image.

- mode-bootloader: Android fastboot mode, it's a mode to re-flash partitions on the Android based device.

- mode-loader: A bootloader mode, it's a mode used to download image on Rockchip platform,

	       usually used in development.

Example:

	reboot-mode {

		mode-normal = <BOOT_NORMAL>;

		mode-recovery = <BOOT_RECOVERY>;

		mode-bootloader = <BOOT_FASTBOOT>;

		mode-loader = <BOOT_BL_DOWNLOAD>;

	}

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>;

		};

	};