dt-bindings: mfd: mc13xxx: Remove obsolete property

The 'fsl,spi-num-chipselects' property is obsolete according to
Documentation/devicetree/bindings/spi/fsl-imx-cspi.txt, so remove
it from the example.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>