dt-bindings: mtd: document Broadcom's BCM47xx partitions

Broadcom based home router devices use partitions which have to be
discovered in a specific way. They are not fixed and there is not any
standard partition table. This commit adds and describes a new custom
binding for such devices.

Signed-off-by: Rafał Miłecki <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Boris Brezillon <[e-mail]>

Documentation/devicetree/bindings/mtd/partition.txt

···
 a subnode of the flash device that is named 'partitions'. It must have a
 'compatible' property, which is used to identify the method to use.
 
-We currently only document a binding for fixed layouts.
+Available bindings are listed in the "partitions" subdirectory.
 
 
 Fixed Partitions

Documentation/devicetree/bindings/mtd/partitions/brcm,bcm947xx-cfe-partitions.txt

···
+Broadcom BCM47xx Partitions
+===========================
+
+Broadcom is one of hardware manufacturers providing SoCs (BCM47xx) used in
+home routers. Their BCM947xx boards using CFE bootloader have several partitions
+without any on-flash partition table. On some devices their sizes and/or
+meanings can also vary so fixed partitioning can't be used.
+
+Discovering partitions on these devices is possible thanks to having a special
+header and/or magic signature at the beginning of each of them. They are also
+block aligned which is important for determinig a size.
+
+Most of partitions use ASCII text based magic for determining a type. More
+complex partitions (like TRX with its HDR0 magic) may include extra header
+containing some details, including a length.
+
+A list of supported partitions includes:
+1) Bootloader with Broadcom's CFE (Common Firmware Environment)
+2) NVRAM with configuration/calibration data
+3) Device manufacturer's data with some default values (e.g. SSIDs)
+4) TRX firmware container which can hold up to 4 subpartitions
+5) Backup TRX firmware used after failed upgrade
+
+As mentioned earlier, role of some partitions may depend on extra configuration.
+For example both: main firmware and backup firmware use the same TRX format with
+the same header. To distinguish currently used firmware a CFE's environment
+variable "bootpartition" is used.
+
+
+Devices using Broadcom partitions described above should should have flash node
+with a subnode named "partitions" using following properties:
+
+Required properties:
+- compatible : (required) must be "brcm,bcm947xx-cfe-partitions"
+
+Example:
+
+flash@0 {
+	partitions {
+		compatible = "brcm,bcm947xx-cfe-partitions";
+	};
+};