dt-bindings: net: dsa: nxp,sja1105: make spi-cpol optional for sja1110

Currently, the binding requires 'spi-cpha' for SJA1105 and 'spi-cpol'
for SJA1110.

However, the SJA1110 supports both SPI modes 0 and 2. Mode 2
(cpha=0, cpol=1) is used by the NXP LX2160 Bluebox 3.

On the SolidRun i.MX8DXL HummingBoard Telematics, mode 0 is stable,
while forcing mode 2 introduces CRC errors especially during bursts.

Drop the requirement on spi-cpol for SJA1110.

Fixes: af2eab1a8243 ("dt-bindings: net: nxp,sja1105: document spi-cpol/cpha")
Signed-off-by: Josua Mayer <josua@solid-run.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20260409-imx8dxl-sr-som-v2-1-83ff20629ba0@solid-run.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>