realtek: pcs: add per-link port storage to rtpcs_serdes

realtek: pcs: add per-link port storage to rtpcs_serdes

Add an s16 link_port[] array to struct rtpcs_serdes, initialised to
-1 in probe. This is preparatory storage for the port number that
each link serves; it will be populated in the follow-up fwnode_pcs
migration commit by scanning consumer DT nodes for their reg, and
consumed by the resolver when allocating rtpcs_link.

Assisted-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Link: https://github.com/openwrt/openwrt/pull/23539
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>