drivers: ethernet: Update Ethernet PHY mode and speed configuration

- Added macros `STM32_ETH_PHY_MODE` to determine
  the PHY mode and speed from the `phy_connection_type`
  property in the device tree.
- Updated the `eth0_data` structure to use `STM32_ETH_PHY_MODE`
  for setting the media interface.

Signed-off-by: IBEN EL HADJ MESSAOUD Marwa <marwa.ibenelhadjmessaoud-ext@st.com>