net: dsa: realtek: convert variants into real drivers

Previously, the interface modules realtek-smi and realtek-mdio served as
a platform and an MDIO driver, respectively. Each interface module
redundantly specified the same compatible strings for both variants and
referenced symbols from the variants.

Now, each variant module has been transformed into a unified driver
serving both as a platform and an MDIO driver. This modification
reverses the relationship between the interface and variant modules,
with the variant module now utilizing symbols from the interface
modules.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>