Multi-Function Devices (MFD)

These devices comprise a nexus for heterogeneous hardware blocks containing
more than one non-unique yet varying hardware functionality.

A typical MFD can be:

- A mixed signal ASIC on an external bus, sometimes a PMIC (Power Management
  Integrated Circuit) that is manufactured in a lower technology node (rough
  silicon) that handles analog drivers for things like audio amplifiers, LED
  drivers, level shifters, PHY (physical interfaces to things like USB or
  ethernet), regulators etc.

- A range of memory registers containing "miscellaneous system registers" also
  known as a system controller "syscon" or any other memory range containing a
  mix of unrelated hardware devices.

Optional properties:

- compatible : "simple-mfd" - this signifies that the operating system should
  consider all subnodes of the MFD device as separate devices akin to how
  "simple-bus" indicates when to see subnodes as children for a simple
  memory-mapped bus. For more complex devices, when the nexus driver has to
  probe registers to figure out what child devices exist etc, this should not
  be used. In the latter case the child devices will be determined by the
  operating system.

- ranges: Describes the address mapping relationship to the parent. Should set
  the child's base address to 0, the physical address within parent's address
  space, and the length of the address map.

- #address-cells: Specifies the number of cells used to represent physical base
  addresses. Must be present if ranges is used.

- #size-cells: Specifies the number of cells used to represent the size of an
  address. Must be present if ranges is used.

Example:

foo@1000 {
	compatible = "syscon", "simple-mfd";
	reg = <0x01000 0x1000>;

	led@8.0 {
		compatible = "register-bit-led";
		offset = <0x08>;
		mask = <0x01>;
		label = "myled";
		default-state = "on";
	};
};