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.org


Signed-off-by: Mark Brown <broonie@kernel.org>