arm: mvebu: Espressobin: Update README.marvell file

Include information about permanent ethernet MAC address and add link how
to build ATF as U-Boot on Espressobin cannot be flashed without ATF.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>
1 file changed