Documentation: devicetree: Update spi cadence bindings doc

Add xilinx compatible string along with cadence.
Compatible string should be first in the node.
Make num-cs optional property.
Add is-decoded-cs property.

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