Device Tree Bindings for Register Bit LEDs

Register bit leds are used with syscon multifunctional devices
where single bits in a certain register can turn on/off a
single LED. The register bit LEDs appear as children to the
syscon device, with the proper compatible string. For the
syscon bindings see:
Documentation/devicetree/bindings/mfd/syscon.txt

Each LED is represented as a sub-node of the syscon device. Each
node's name represents the name of the corresponding LED.

LED sub-node properties:

Required properties:
- compatible : must be "register-bit-led"
- offset : register offset to the register controlling this LED
- mask : bit mask for the bit controlling this LED in the register
  typically 0x01, 0x02, 0x04 ...

Optional properties:
- label : (optional)
  see Documentation/devicetree/bindings/leds/common.txt
- linux,default-trigger : (optional)
  see Documentation/devicetree/bindings/leds/common.txt
- default-state: (optional) The initial state of the LED. Valid
  values are "on", "off", and "keep". If the LED is already on or off
  and the default-state property is set the to same value, then no
  glitch should be produced where the LED momentarily turns off (or
  on). The "keep" setting will keep the LED at whatever its current
  state is, without producing a glitch.  The default is off if this
  property is not present.

Example:

syscon: syscon@10000000 {
	compatible = "arm,realview-pb1176-syscon", "syscon";
	reg = <0x10000000 0x1000>;

	led@08.0 {
		compatible = "register-bit-led";
		offset = <0x08>;
		mask = <0x01>;
		label = "versatile:0";
		linux,default-trigger = "heartbeat";
		default-state = "on";
	};
	led@08.1 {
		compatible = "register-bit-led";
		offset = <0x08>;
		mask = <0x02>;
		label = "versatile:1";
		linux,default-trigger = "mmc0";
		default-state = "off";
	};
	led@08.2 {
		compatible = "register-bit-led";
		offset = <0x08>;
		mask = <0x04>;
		label = "versatile:2";
		linux,default-trigger = "cpu0";
		default-state = "off";
	};
	led@08.3 {
		compatible = "register-bit-led";
		offset = <0x08>;
		mask = <0x08>;
		label = "versatile:3";
		default-state = "off";
	};
	led@08.4 {
		compatible = "register-bit-led";
		offset = <0x08>;
		mask = <0x10>;
		label = "versatile:4";
		default-state = "off";
	};
	led@08.5 {
		compatible = "register-bit-led";
		offset = <0x08>;
		mask = <0x20>;
		label = "versatile:5";
		default-state = "off";
	};
	led@08.6 {
		compatible = "register-bit-led";
		offset = <0x08>;
		mask = <0x40>;
		label = "versatile:6";
		default-state = "off";
	};
	led@08.7 {
		compatible = "register-bit-led";
		offset = <0x08>;
		mask = <0x80>;
		label = "versatile:7";
		default-state = "off";
	};
};