realtek: DTS: add macro for switch port with SerDes

realtek: DTS: add macro for switch port with SerDes

In the future the PCS & DSA drivers will lookup the SerDes of a
switch port via pcs-handle (like upstream does). Provide a macro
that allows to expand the existing port definitions. To link a
SerDes to port simply do

Either in short form:

replace SWITCH_PORT(0, 1, qsgmii)
with    SWITCH_PORT_SDS(0, 1, 3, qsgmii) (Link to SerDes 3)

Or in long form:

port@24 {
	reg = <24>;
	label = "lan25";
	pcs-handle = <&serdes4>; (Link to SerDes 4)
	phy-handle = <&phy24>;
	phy-mode = "1000base-x";
	managed = "in-band-status";
	sfp = <&sfp0>;
};

Signed-off-by: Markus Stockhausen <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20075
Signed-off-by: Robert Marko <[email protected]>