arm: mvebu: Espressobin: Instructions for more MAC addresses in README.marvell

Some Espressobin boards got assigned more than one MAC address. Update
instructions how to correctly store and preserve all MAC addresses.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Andre Heider <a.heider@gmail.com>
1 file changed