net: ethernet: dwmac: add Ethernet glue logic for stm32 chip

stm324xx family chips support Synopsys MAC 3.510 IP.
This patch adds settings for logical glue logic:
-clocks
-mode selection MII or RMII.

Reviewed-by: Joachim Eastwood <manabian@gmail.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Tested-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Alexandre TORGUE <alexandre.torgue@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>