dt: fsl-imx-cspi.txt: Fix comment about GPIOs used for chip selects

The gpio numbers does not match the comment.

Fix the comments for GPIOs used for chip selects.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>