dt-bindings: reset: Add document for Broadcom STB reset controller

Add a binding document for the Broadcom STB reset controller, also known
as SW_INIT-style reset controller.
Signed-off-by: NFlorian Fainelli <f.fainelli@gmail.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NPhilipp Zabel <p.zabel@pengutronix.de>

Documentation/devicetree/bindings/reset/brcm,brcmstb-reset.txt

+Broadcom STB SW_INIT-style reset controller
+===========================================
+
+Broadcom STB SoCs have a SW_INIT-style reset controller with separate
+SET/CLEAR/STATUS registers and possibly multiple banks, each of 32 bit
+reset lines.
+
+Please also refer to reset.txt in this directory for common reset
+controller binding usage.
+
+Required properties:
+- compatible: should be brcm,brcmstb-reset
+- reg: register base and length
+- #reset-cells: must be set to 1
+
+Example:
+
+	reset: reset-controller@8404318 {
+		compatible = "brcm,brcmstb-reset";
+		reg = <0x8404318 0x30>;
+		#reset-cells = <1>;
+	};
+
+	&ethernet_switch {
+		resets = <&reset>;
+		reset-names = "switch";
+	};