dt-bindings: mtd: add a common label property to all mtd devices

This can be used to easily identify a specific chip on a system with
multiple chips.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>