ARM: dts: a20-wits-pro-a20-dkt: Enable AHCI

Enable ahci node for a20-wits-pro-a20-dkt, this would require since
we have DM_SCSI enabled on the respective SoC.

Right now, ahci enabled in -u-boot.dtsi and will remove once same
supported by Linux.

Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>