qca-ssdk: support selecting PCS channel for PORT3 on IPQ6018

qca-ssdk: support selecting PCS channel for PORT3 on IPQ6018

When QCA8072 is used in PSGMII mode with IPQ6018, PCS used for second
PHY port would overlap with one used by SGMII+ port. SoC has register
to select different PCS in such case.

Original code used PHY_ID for this decision, which also had other
issues, but is no longer viable since we moved to upstream QCA807x
driver.

Introduce DT property port3_pcs_channel to allow describing this in DT.
Default value is <2>, and for some QCA8072 designs <4> would be needed.

Signed-off-by: Mantas Pucka <mantas@8devices.com>