arm: armada-xp: Add SPL support used to include the DDR training code

This patch adds SPL support to the Marvell Armada-XP. With this addition
the bin_hdr integration is not needed any more. The SPL will first
initialize the serdes/PHY and the call the DDR setup and training code
now integrated into mainline U-Boot.

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