net: mdio: realtek-rtl9300: relocate c22/c45 device tree readout

otto_emdio_map_ports() is the central place to lookup the topology and the
properties of the Realtek ethernet MDIO controller from the device tree.
Deviating from this the c22/c45 detection via "ethernet-phy-ieee802.3-c45"
is running separately in otto_emdio_probe_one(). It loops over the same
nodes, just at a later point in time.

There is no benefit to divide this setup and to have a time window where
the data structure is only filled partially. Additionally it uses the
"fwnode" API. Consolidate the setup and convert it to the "of" API.

Remark. This is a subtle change for dangling PHY nodes (not referenced
by ethernet-ports). Before this commit all PHY nodes were evaluated for
c45 setup, now only the referenced ones.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://patch.msgid.link/20260603175924.123019-6-markus.stockhausen@gmx.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>