ARM: dts: a20-m5: Enable AHCI

Enable ahci node for sun7i-a20-m5.dts, this would require since
we have DM_SCSI enabled on the respective SoC.

No need to send patch to Linux for this change, since this
dts is U-Boot specific.

Cc: Ian Campbell <ijc@hellion.org.uk>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>