spi: Fix the number of CS lines documented as an example

The number of CS lines is mentioned as 2 in the spi-controller binding
but however in the example, 4 cs-gpios are used. Hence fix that to
mention 4.
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20190820115000.32041-1-manivannan.sadhasivam@linaro.orgSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-controller.yaml

@@ -31,7 +31,7 @@ properties:
       If that property is used, the number of chip selects will be
       increased automatically with max(cs-gpios, hardware chip selects).
 
-      So if, for example, the controller has 2 CS lines, and the
+      So if, for example, the controller has 4 CS lines, and the
       cs-gpios looks like this
         cs-gpios = <&gpio1 0 0>, <0>, <&gpio1 1 0>, <&gpio1 2 0>;