mtd: partitions: Add RedBoot FIS DT bindings

This adds device tree bindings for the RedBoot FIS partition
format.

Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>

Documentation/devicetree/bindings/mtd/partitions/redboot-fis.txt

+RedBoot FLASH Image System (FIS) Partitions
+===========================================
+
+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>;
+	};
+};