realtek: dts: add link index cell to pcs-handle phandles

realtek: dts: add link index cell to pcs-handle phandles

A SerDes can host multiple PCS links: QSGMII binds four ports to one
SerDes, USXGMII variants up to eight. Today pcs-handle references the
SerDes as a whole, with no way to express which link inside the SerDes
a port wants. The driver gets away with this because it carries its own
port->link bookkeeping and the link slot is implicit in DSA's port
iteration order -- functional, but the wiring information lives nowhere
in DT.

The upcoming fwnode_pcs migration moves PCS lookup to the generic
fwnode provider API, which disambiguates multiple instances per fwnode
via phandle cells. To make that landable as small, code-only commits,
the DT needs to carry the link index ahead of time.

Bump #pcs-cells from 0 to 1 on every SerDes node in the four SoC DTSIs
and append the link cell to every pcs-handle reference across boards
and the SWITCH_PORT_* macros. Cell values match the existing wiring:
0 for single-link SerDes (10GBase-R, SGMII, fiber, single-link
USXGMII), 0..3 per SerDes for QSGMII and USXGMII-QX, 0..7 for the
RTL9311 octal USXGMII layout.

No code reads the new cell yet -- of_parse_phandle_with_args() in the
PCS driver already cooperates with cells = 0 or 1, and the DSA glue
uses of_parse_phandle() which ignores cells entirely. The change is
runtime-neutral on its own; it exists so the follow-up code patches
can be a few lines each instead of dragging a bridge counter into the
driver to invent slot numbers DT could have provided directly.

Link: https://github.com/openwrt/openwrt/pull/23539
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>