spi: spi-fsl-lpspi: add num-cs binding for lpspi

Add num-cs property to support multiple cs for lpspi. This property is
optional.

Signed-off-by: Han Xu <[e-mail]>
Acked-by: Krzysztof Kozlowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/spi/spi-fsl-lpspi.yaml

···
       this property to re-config the chipselect value in the LPSPI driver.
     type: boolean
 
+  num-cs:
+    description:
+      number of chip selects.
+    minimum: 1
+    maximum: 2
+    default: 1
+
 required:
   - compatible
   - reg
···
         clock-names = "per", "ipg";
         spi-slave;
         fsl,spi-only-use-cs1-sel;
+        num-cs = <2>;
     };