net: ftgmac100: Add DM_MDIO support

Add support for DM_MDIO to connect to PHY.  For the systems that have a
stand-alone MDIO hardware block, enable CONFIG_DM_MDIO to use driver
model for MDIO devices.

Signed-off-by: Dylan Hung <dylan_hung@aspeedtech.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>