net: phy: Introduce PSGMII PHY interface mode

The PSGMII interface is similar to QSGMII. The main difference
is that the PSGMII interface combines five SGMII lines into a
single link while in QSGMII only four lines are combined.

Similarly to the QSGMII, this interface mode might also needs
special handling within the MAC driver.

It is commonly used by Qualcomm with their QCA807x PHY series and
modern WiSoC-s.

Add definitions for the PHY layer to allow to express this type
of connection between the MAC and PHY.

Signed-off-by: Gabor Juhos <j4g8y7@gmail.com>
Signed-off-by: Robert Marko <robert.marko@sartura.hr>
Signed-off-by: David S. Miller <davem@davemloft.net>