mfd: syscon: Add Atmel Matrix bus DT binding documentation

The Matrix registers are provided to configure internal bus behavior on
at91 SoCs.
Some registers might be accessed by several drivers (e.g. to configure
external memory bus timings), hence we declare this register set as a
syscon device.
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>

Documentation/devicetree/bindings/mfd/atmel-matrix.txt

+* Device tree bindings for Atmel Bus Matrix
+
+The Bus Matrix registers are used to configure Atmel SoCs internal bus
+behavior (master/slave priorities, undefined burst length type, ...)
+
+Required properties:
+- compatible:		Should be one of the following
+			"atmel,at91sam9260-matrix", "syscon"
+			"atmel,at91sam9261-matrix", "syscon"
+			"atmel,at91sam9263-matrix", "syscon"
+			"atmel,at91sam9rl-matrix", "syscon"
+			"atmel,at91sam9g45-matrix", "syscon"
+			"atmel,at91sam9n12-matrix", "syscon"
+			"atmel,at91sam9x5-matrix", "syscon"
+			"atmel,sama5d3-matrix", "syscon"
+- reg:			Contains offset/length value of the Bus Matrix
+			memory region.
+
+Example:
+
+matrix: matrix@ffffec00 {
+	compatible = "atmel,sama5d3-matrix", "syscon";
+	reg = <0xffffec00 0x200>;
+};