arm: mvebu: Add Serdes PHY config code

This code is ported from the Marvell bin_hdr code into mainline
SPL U-Boot. It needs to be executed very early so that the devices
connected to the serdes PHY are configured correctly.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Luka Perkov <luka.perkov@sartura.hr>