arm: mvebu: drivers/ddr: Move Armada XP DDR init code into new directory

With the upcoming addition of the Armada 38x DDR support, which is not
compatible to the Armada XP DDR init code, we need to introduce a new
directory infrastructure. To support multiple Marvell DDR controller.

This will be the new structure:

     drivers/ddr/marvell/axp
     Supporting Armada XP (AXP) devices (and perhaps Armada 370)

     drivers/ddr/marvell/a38x
     Supporting Armada 38x devices (and perhaps Armada 39x)

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