arm: mvebu: db-88f6820: Add SPL support with DDR init code

This patch adds SPL support for the Marvell DB-88F6820-GP board.
With this change, the bin_hdr from the original Marvell U-boot
is not needed any more on this board. The sources from bin_hdr
(SERDES/PHY and DDR setup) are now integrated in mainline
U-Boot. And this patch enables them for this board.

Signed-off-by: Stefan Roese <sr@denx.de>