mdio bus isn't a standalone device on ar7240. (and maybe older SoCs?)
Use simple-mfd for ar7241 and later SoCs to get mdio1 ready before gmac0
For ar7240 and older chips, manually create platform device after
ag71xx_hw_init() in ag71xx_probe()to get mdio0 ready between
ag71xx_hw_init() and ag71xx_phy_connect().
Signed-off-by: Chuanhong Guo <gch981213@gmail.com>