dt-bindings: fsl-quadspi: Add the example of two SPI NOR

Improve the bindings example by adding an example of how to represent
two SPI NOR devices.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Han Xu <han.xu@nxp.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>