realtek: dsa: allow to drop phy-handle on switch ports

realtek: dsa: allow to drop phy-handle on switch ports

When Realtek SerDes is completely handled by PCS, it is not treated as
a regular PHY anymore. Thus, we should be able to drop the currently
used pseudo-PHYs and phy-handles for ports which just use the SerDes as
PCS but have no PHY attached.

Allow to drop the phy-handle from switch port definitions if there is a
pcs-handle defined by relaxing several checks in the DSA driver.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/20577
Signed-off-by: Robert Marko <robimarko@gmail.com>