MFD/OF: document MFD devices and handle simple-mfd

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" inidicates 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.

Example:

foo@1000 {

	compatible = "syscon", "simple-mfd";

	reg = <0x01000 0x1000>;

	led@08.0 {

		compatible = "register-bit-led";

		offset = <0x08>;

		mask = <0x01>;

		label = "myled";

		default-state = "on";

	};

};

const struct of_device_id of_default_bus_match_table[] = {

	{ .compatible = "simple-bus", },

	{ .compatible = "simple-mfd", },

#ifdef CONFIG_ARM_AMBA

	{ .compatible = "arm,amba-bus", },

#endif /* CONFIG_ARM_AMBA */