devicetree:bindings: add devicetree bindings for Freescale AHCI

adds bindings for Freescale QorIQ AHCI SATA controller.
Signed-off-by: Tang Yuantian <Yuantian.Tang@freescale.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Documentation/devicetree/bindings/ata/ahci-fsl-qoriq.txt

+Binding for Freescale QorIQ AHCI SATA Controller
+
+Required properties:
+  - reg: Physical base address and size of the controller's register area.
+  - compatible: Compatibility string. Must be 'fsl,<chip>-ahci', where
+    chip could be ls1021a, ls2085a, ls1043a etc.
+  - clocks: Input clock specifier. Refer to common clock bindings.
+  - interrupts: Interrupt specifier. Refer to interrupt binding.
+
+Optional properties:
+  - dma-coherent: Enable ACHI coherency DMA operation.
+  - reg-names: register area names when there are more then 1 regster area.
+
+Examples:
+	sata@3200000 {
+		compatible = "fsl,ls1021a-ahci";
+		reg = <0x0 0x3200000 0x0 0x10000>;
+		interrupts = <GIC_SPI 101 IRQ_TYPE_LEVEL_HIGH>;
+		clocks = <&platform_clk 1>;
+		dma-coherent;
+	};