net: phy: realtek: merge the drivers for internal NBase-T PHY's

net: phy: realtek: merge the drivers for internal NBase-T PHY's

The Realtek RTL8125/RTL8126 NBase-T MAC/PHY chips have internal PHY's
which are register-compatible, at least for the registers we use here.
So let's use just one PHY driver to support all of them.
These internal PHY's exist also as external C45 PHY's, but on the
internal PHY's no access to MMD registers is possible. This can be
used to differentiate between the internal and external version.

As a side effect the drivers for two now external-only drivers don't
require read_mmd/write_mmd hooks any longer.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://patch.msgid.link/c57081a6-811f-4571-ab35-34f4ca6de9af@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>