Virtual MTD concat device

Requires properties:
- devices: list of phandles to mtd nodes that should be concatenated

Example:

&spi {
	flash0: flash@0 {
		...
	};
	flash1: flash@1 {
		...
	};
};

flash {
	compatible = "mtd-concat";

	devices = <&flash0 &flash1>;

	partitions {
		compatible = "fixed-partitions";

		partition@0 {
			label = "boot";
			reg = <0x0000000 0x0040000>;
			read-only;
		};

		partition@40000 {
			label = "firmware";
			reg = <0x0040000 0x1fc0000>;
		};
	}
}