dt-bindings: ata: Document BCM7216 AHCI controller compatible

The BCM7216 AHCI controller makes use of a specific reset controller
line/name, document the compatible string and the optional reset
properties.

Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>