generic: ar8216: add support for separated mdio bus for phy access

generic: ar8216: add support for separated mdio bus for phy access

Atheros FE switches have a builtin mdio master available for PHY
accessing and on ar724x/ar933x builtin switches this mdio master
is the only way of accessing PHYs.

After this patch if there is phy_read/phy_write method available
in ar8xxx_chip we register a separated mdio bus for accessing PHYs.

Still adds support for mdio device probing only since this isn't
needed for those switches registered using PHY probing.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>