ipq40xx: add PSGMII PHY mode to phylink_get_linkmodes()

ipq40xx: add PSGMII PHY mode to phylink_get_linkmodes()

Upstream commit ("net: phylink: add generic validate implementation") was
backported, however PSGMII PHY mode patch for ipq40xx was not updated to
add PSGMII to phylink_get_linkmodes() so the following warning would be
printed during kernel compilation:
drivers/net/phy/phylink.c: In function 'phylink_get_linkmodes':
drivers/net/phy/phylink.c:360:9: error: enumeration value 'PHY_INTERFACE_MODE_PSGMII' not handled in switch [-Werror=switch]
  360 |         switch (interface) {
      |         ^~~~~~

Resolve the warning by adding the PSGMII mode to phylink_get_linkmodes().

Signed-off-by: Robert Marko <robimarko@gmail.com>