arm: mvebu: Add SATA/SCSI (AHCI) support for Armada A38x

This patch adds support for the common AHCI controller on the Marvell
Armada 38x.

Tested on the Marvell DB-88F6820-GP eval board.

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