ARM64: zynqmp: Enable AHCI on EP platform

The EP platform also has working AHCI emulation, so I see little reason
not to implement the plumbing for it that enables us to boot from AHCI.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>