net: ftgmac100: add handling of mdio/phy nodes for ast2400/2500

phy-handle can't be handled well for ast2400/2500 which has an embedded
MDIO controller. Add ftgmac100_mdio_setup for ast2400/2500 and initialize
PHYs from mdio child node with of_mdiobus_register.

Signed-off-by: Ivan Mikhaylov <i.mikhaylov@yadro.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>