net: dsa: qca8k: rework and simplify mdiobus logic

In an attempt to reduce qca8k_priv space, rework and simplify mdiobus
logic.
We now declare a mdiobus instead of relying on DSA phy_read/write even
if a mdio node is not present. This is all to make the qca8k ops static
and not switch specific. With a legacy implementation where port doesn't
have a phy map declared in the dts with a mdio node, we declare a
'qca8k-legacy' mdiobus. The conversion logic is used as legacy read and
write ops are used instead of the internal one.
Also drop the legacy_phy_port_mapping as we now declare mdiobus with ops
that already address the workaround.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>