ARM: dts: imx: remove obsoleted property fsl,spi-num-chipselects

Since commit b36581df ("spi: imx: Using existing properties for
chipselects") the device tree property 'fsl,spi-num-chipselects' is
unused and it is already marked as obsolete in device tree binding
documentation. Remove the property from the existing DTS files to
avoid its reoccurence on copying.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>