boards: Specify PHY connection type

This commit updates the device tree source files
for all STM32 boards to specify the PHY connection
type for the Ethernet configuration.
Added the `phy-connection-type` property
with the value "rmii" to the Ethernet node.

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