mtd: partitions: Add RedBoot FIS DT bindings

RedBoot FLASH Image System (FIS) Partitions

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The FLASH Image System (FIS) directory is a flash description

format closely associated with the RedBoot boot loader.

It uses one single flash eraseblock in the flash to store an index of

all images in the flash.

This block size will vary depending on flash but is typically

32 KB in size.

Required properties:

- compatible : (required) must be "redboot-fis"

- fis-index-block : (required) a index to the eraseblock containing

  the FIS directory on this device. On a flash memory with 32KB

  eraseblocks, 0 means the first eraseblock at 0x00000000, 1 means the

  second eraseblock at 0x00008000 and so on.

Example:

flash@0 {

	partitions {

		compatible = "redboot-fis";

		fis-index-block = <0>;

	};

};