ARM: dts: imx53-ppd: Remove 'num-chipselects' property

The 'num-chipselects' property is not a valid property according
to Documentation/devicetree/bindings/spi/fsl-imx-cspi.txt, so
let's remove it.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>