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>

Documentation/devicetree/bindings/mtd/nand.txt

@@ -43,6 +43,7 @@ Optional NAND chip properties:
 		     This is particularly useful when only the in-band area is
 		     used by the upper layers, and you want to make your NAND
 		     as reliable as possible.
+- nand-rb: shall contain the native Ready/Busy ids.
 
 The ECC strength and ECC step size properties define the correction capability
 of a controller. Together, they say a controller can correct "{strength} bit