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>

Documentation/devicetree/bindings/ata/ahci-dm816.txt

+Device tree binding for the TI DM816 AHCI SATA Controller
+---------------------------------------------------------
+
+Required properties:
+  - compatible: must be "ti,dm816-ahci"
+  - reg: physical base address and size of the register region used by
+         the controller (as defined by the AHCI 1.1 standard)
+  - interrupts: interrupt specifier (refer to the interrupt binding)
+  - clocks: list of phandle and clock specifier pairs (or only
+            phandles for clock providers with '0' defined for
+            #clock-cells); two clocks must be specified: the functional
+            clock and an external reference clock
+
+Example:
+
+	sata: sata@4a140000 {
+		compatible = "ti,dm816-ahci";
+		reg = <0x4a140000 0x10000>;
+		interrupts = <16>;
+		clocks = <&sysclk5_ck>, <&sata_refclk>;
+	};