dt-bindings: net: micrel,ks8851: allow SPI device properties

The Micrel KS8851 can be attached to SPI or parallel bus and the
difference is expressed in compatibles.  Allow common SPI properties
when this is a SPI variant and narrow the parallel memory bus properties
to the second case.

This fixes dtbs_check warning:

  qcom-msm8960-cdp.dtb: ethernet@0: Unevaluated properties are not allowed ('spi-max-frequency' was unexpected)

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230619170134.65395-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>