ARM: stm32: Allow overriding setup_mac_address()

Let board code override setup_mac_address(), which is useful e.g. if the
board derives the MAC address from another source, like an I2C EEPROM.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>