drivers: ethernet: Integrate GRMII and RMII interfaces

- Added macros `STM32_ETH_PHY_MODE`
  to determine the PHY mode (RGMII, GMII, RMII, and MII)
  from the `phy_connection_type` property in the device tree.
- Removed previous definitions for ETH_MEDIA_INTERFACE_MII
  and ETH_MEDIA_INTERFACE_RMII.
- Updated STM32_ETH_PHY_MODE macro to use ETH_MII_MODE
  and ETH_RMII_MODE.

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