ARM: dts: stm32: replace rgmii mode with rgmii-id on stm32mp15 boards

On disco and eval board, Tx and Rx delay are applied (pull-up of 4.7k
put on VDD) so which correspond to RGMII-ID mode with internal RX and TX
delays provided by the PHY, the MAC should not add the RX or TX delays
in this case

Signed-off-by: Christophe Roullier <christophe.roullier@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>