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

This defines a new compatible option for MFD devices "simple-mfd" that will
make the OF core spawn child devices for all subnodes of that MFD device.
It is optional but handy for things like syscon and possibly other
simpler MFD devices.

Since there was no file to put the documentation in, I took this opportunity
to make a small writeup on MFD devices and add the compatible definition
there.
Suggested-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Devicetree <devicetree@vger.kernel.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Grant Likely <grant.likely@linaro.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/mfd/mfd.txt

+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";
+	};
+};

drivers/of/platform.c

@@ -25,6 +25,7 @@
 
 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 */