smt32mp: add setup_mac_address for stm32mp25

Add a function setup_mac_address() to update the MAC address from the
default location in OTP for stm32mp2 platform.

The max number of OTP for MAC address is increased to 8 for STM32MP25,
defined with get_eth_nb() and checked in setup_mac_address.

The MAC address FF:FF:FF:FF:FF:FF, the broadcast ethaddr, is a invalid
value used for unused MAC address slot in OTP, for example for board
with STM32MP25x part number allows up to 5 ethernet ports but it is not
supported by the hardware, without switch; the associated variable
"enetaddr%d" is not created.

Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>