Documentation: bindings: document the sub-nodes AHCI bindings

The libahci now allows to use multiple PHYs and to represent each port
as a sub-node. Add these bindings to the documentation.
Signed-off-by: Antoine Ténart <antoine.tenart@free-electrons.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

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

@@ -3,6 +3,10 @@
 SATA nodes are defined to describe on-chip Serial ATA controllers.
 Each SATA controller should have its own node.
 
+It is possible, but not required, to represent each port as a sub-node.
+It allows to enable each port independently when dealing with multiple
+PHYs.
+
 Required properties:
 - compatible        : compatible string, one of:
   - "allwinner,sun4i-a10-ahci"
@@ -12,13 +16,30 @@ Required properties:
   - "snps,dwc-ahci"
   - "snps,exynos5440-ahci"
   - "snps,spear-ahci"
+  - "generic-ahci"
 - interrupts        : <interrupt mapping for SATA IRQ>
 - reg               : <registers mapping>
 
+Please note that when using "generic-ahci" you must also specify a SoC specific
+compatible:
+	compatible = "manufacturer,soc-model-ahci", "generic-ahci";
+
 Optional properties:
 - dma-coherent      : Present if dma operations are coherent
 - clocks            : a list of phandle + clock specifier pairs
 - target-supply     : regulator for SATA target power
+- phys              : reference to the SATA PHY node
+- phy-names         : must be "sata-phy"
+
+Required properties when using sub-nodes:
+- #address-cells    : number of cells to encode an address
+- #size-cells       : number of cells representing the size of an address
+
+
+Sub-nodes required properties:
+- reg               : the port number
+- phys              : reference to the SATA PHY node
+
 
 Examples:
         sata@ffe08000 {
@@ -34,3 +55,23 @@ Examples:
 		clocks = <&pll6 0>, <&ahb_gates 25>;
 		target-supply = <&reg_ahci_5v>;
 	};
+
+With sub-nodes:
+	sata@f7e90000 {
+		compatible = "marvell,berlin2q-achi", "generic-ahci";
+		reg = <0xe90000 0x1000>;
+		interrupts = <GIC_SPI 7 IRQ_TYPE_LEVEL_HIGH>;
+		clocks = <&chip CLKID_SATA>;
+		#address-cells = <1>;
+		#size-cells = <0>;
+
+		sata0: sata-port@0 {
+			reg = <0>;
+			phys = <&sata_phy 0>;
+		};
+
+		sata1: sata-port@1 {
+			reg = <1>;
+			phys = <&sata_phy 1>;
+		};
+	};