ata: ahci_brcm: Add a shutdown callback

Make sure that we quiesce the controller and shut down the clocks in a
shutdown callback.

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>