drivers: ethernet: Update STM32 Ethernet driver to support MDIO if enabled.

Moved the definition of the phy device from depending on stm32hx
series SOC to its own #if...#else...#endif. This way mdio support
can be enabled on any stm32 soc that supports it by defining the
mdio node in the devicetree.

I have tested this in a custom board.

Signed-off-by: Rahul Arasikere <arasikere.rahul@gmail.com>