dt-bindings: mtd: document new nand-rb property

There are already an atmel,rb and an allwinner,rb properties, let's not
make other ones and instead use a generic term: nand-rb to define NAND
chips Ready/Busy lines.

Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>