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

Broadcom TRX Container Partition

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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";

	};

};