ARM: stm32: DH: Use common MAC address functions

To reduce code duplication, let the stm32 based DH boards use the common
code for setting up their MAC addresses.

Signed-off-by: Philip Oberfichtner <pro@denx.de>
Tested-by: Marek Vasut <marex@denx.de>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>