dt-bindings: net: nxp,sja1105: document spi-cpol/cpha

Some boards use SJA1105 Ethernet Switch with SPI CPHA, while ones with
SJA1110 use SPI CPOL, so document this to fix dtbs_check warnings:

  arch/arm64/boot/dts/freescale/fsl-lx2160a-bluebox3.dtb: ethernet-switch@0: Unevaluated properties are not allowed ('spi-cpol' was unexpected)

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>