spi: cadence: Chip select related changes

Change chip select property name to "num-cs". Use a default value
instead of giving error when num-cs is not found.
Add "is-decoded-cs" property for using extended slave select; make
necessary changes in cdns_spi_chipselect function.

Signed-off-by: Harini Katakam <harinik@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>