dt-bindings: ata: Convert apm,xgene-ahci to DT schema

Convert the APM X-Gene AHCI SATA Controller to DT schema format. It's a
straight-forward conversion.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>