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 <han.xu@nxp.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221206225410.604482-2-han.xu@nxp.comSigned-off-by: Mark Brown <broonie@kernel.org>

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

@@ -56,6 +56,13 @@ properties:
       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
@@ -80,4 +87,5 @@ examples:
         clock-names = "per", "ipg";
         spi-slave;
         fsl,spi-only-use-cs1-sel;
+        num-cs = <2>;
     };