dt-bindings: mtd: document Broadcom's TRX firmware format binding

Broadcom based home router devices use TRX firmware format. It's a
container that can hold few partitions. If operating system is expected
to access them it has to understand that format and parse flash data
accordingly.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>

Documentation/devicetree/bindings/mtd/partitions/brcm,trx.txt

+Broadcom TRX Container Partition
+================================
+
+TRX is Broadcom's official firmware format for the BCM947xx boards. It's used by
+most of the vendors building devices based on Broadcom's BCM47xx SoCs and is
+supported by the CFE bootloader.
+
+Design of the TRX format is very minimalistic. Its header contains
+identification fields, CRC32 checksum and the locations of embedded partitions.
+Its purpose is to store a few partitions in a format that can be distributed as
+a standalone file and written in a flash memory.
+
+Container can hold up to 4 partitions. The first partition has to contain a
+device executable binary (e.g. a kernel) as it's what the CFE bootloader starts
+executing. Other partitions can be used for operating system purposes. This is
+useful for systems that keep kernel and rootfs separated.
+
+TRX doesn't enforce any strict partition boundaries or size limits. All
+partitions have to be less than the 4GiB max size limit.
+
+There are two existing/known TRX variants:
+1) v1 which contains 3 partitions
+2) v2 which contains 4 partitions
+
+There aren't separated compatible bindings for them as version can be trivialy
+detected by a software parsing TRX header.
+
+Required properties:
+- compatible : (required) must be "brcm,trx"
+
+Example:
+
+flash@0 {
+	partitions {
+		compatible = "brcm,trx";
+	};
+};