ARM: dts: stm32: Update eth1addr from EEPROM if eth1 present

The STM32MP1 DHCOM has two ethernet interfaces, the on-SoM DWMAC and KS8851.
Set eth1addr for the KS8851 to a MAC address of the DWMAC incremented by 1.
The MAC of the DWMAC is set from on-SoM EEPROM already, but the MAC address
of KS8851 was left uninitialized, so fix this.

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