Document existing SPI dev compatibles.

Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230120075618.153664-2-krzysztof.kozlowski@linaro.orgSigned-off-by: Mark Brown <broonie@kernel.org>