dts: spi: Add optional cs-gpios to the spi bus controller binding

Now that we can generate gpio info from dts lets add optional support
fot the cs-gpios property in the bus controller node.  This matches the
binding spec from Linux on how cs-gpios are handled.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>

dts/bindings/spi/spi.yaml

@@ -28,4 +28,11 @@ properties:
       category: required
       description: Human readable string describing the device (used by Zephyr for API name)
       generation: define
+
+    cs-gpios:
+      type: compound
+      category: optional
+      generation: define, use-prop-name
+
+
 ...