ARM: dts: stm32: Rename mdio0 to mdio on DHCOR Testbench board

Replace "mdio0" node with "mdio" to match mdio.yaml DT schema.

Fixes: c8ce0dd7 ("ARM: dts: stm32: Add DHCOR based Testbench board")
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>