libata, dt: Document sata_rcar bindings

These bindings can be used to register SATA devices found on R-Car SoC.
Signed-off-by: NValentine Barshak <valentine.barshak@cogentembedded.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

Documentation/devicetree/bindings/ata/sata_rcar.txt

+* Renesas R-Car SATA
+
+Required properties:
+- compatible		: should contain one of the following:
+			  - "renesas,sata-r8a7779" for R-Car H1
+			  - "renesas,sata-r8a7790" for R-Car H2
+			  - "renesas,sata-r8a7791" for R-Car M2
+- reg			: address and length of the SATA registers;
+- interrupts		: must consist of one interrupt specifier.
+
+Example:
+
+sata: sata@fc600000 {
+	compatible = "renesas,sata-r8a7779";
+	reg = <0xfc600000 0x2000>;
+	interrupt-parent = <&gic>;
+	interrupts = <0 100 IRQ_TYPE_LEVEL_HIGH>;
+};