dt-bindings: Document the DesignWare IP reset bindings

This adds documentation of device tree bindings for the
DesignWare IP reset controller.
Signed-off-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Signed-off-by: Luis Oliveira <luis.oliveira@synopsys.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

Documentation/devicetree/bindings/reset/snps,dw-reset.txt

+Synopsys DesignWare Reset controller
+=======================================
+
+Please also refer to reset.txt in this directory for common reset
+controller binding usage.
+
+Required properties:
+
+- compatible: should be one of the following.
+	"snps,dw-high-reset" - for active high configuration
+	"snps,dw-low-reset" - for active low configuration
+
+- reg: physical base address of the controller and length of memory mapped
+	region.
+
+- #reset-cells: must be 1.
+
+example:
+
+	dw_rst_1: reset-controller@0000 {
+		compatible = "snps,dw-high-reset";
+		reg = <0x0000 0x4>;
+		#reset-cells = <1>;
+	};
+
+	dw_rst_2: reset-controller@1000 {i
+		compatible = "snps,dw-low-reset";
+		reg = <0x1000 0x8>;
+		#reset-cells = <1>;
+	};