dt-binding: ata: tegra: Add dt-binding documentation for Tegra186

This patch adds dt-bindings documentation for Tegra186 AHCI
controller.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sowjanya Komatineni <skomatineni@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/1617758731-12380-3-git-send-email-skomatineni@nvidia.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>