spi: dt-bindings: spi-controller: correct example indentation

Example DTS mixed two with four-space indentation.  Preferred is four
spaces, for readability.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220722190910.76865-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>