dt-bindings: ata: ahci-platform: document ahci-supply

This patch document the new optional ahci-supply.

Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>