dt-bindings: ata: add DT bindings for ahci-dm816 SATA controller

Add DT bindings for the onboard SATA controller present on the DM816x
SoCs.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>