Documentation: fsl-quadspi: Add an entry for the imx6sx compatible string

"fsl,imx6sx-qspi" is also a valid compatible string, so add an entry for it.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>