ARC: reset: add missing DT binding documentation for HSDKv1 reset driver

When applying the original patch [1], the DT binding docs were lost.
This patch adds them back.

[1] https://patchwork.kernel.org/patch/9852997/

Fixes: e0be864f ("ARC: reset: introduce HSDKv1 reset driver")
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

Documentation/devicetree/bindings/reset/snps,hsdk-v1-reset.txt

+Binding for the HSDK v1 reset controller
+
+This binding uses the common reset binding[1].
+
+[1] Documentation/devicetree/bindings/reset/reset.txt
+
+Required properties:
+- compatible: should be "snps,hsdk-v1.0-reset".
+- reg: should always contain 2 pairs address - length: first for reset
+  configuration register and second for corresponding SW reset and status bits
+  register.
+- #reset-cells: from common reset binding; Should always be set to 1.
+
+Example:
+	reset: reset@880 {
+		compatible = "snps,hsdk-v1.0-reset";
+		#reset-cells = <1>;
+		reg = <0x8A0 0x4>, <0xFF0 0x4>;
+	};
+
+Specifying reset lines connected to IP modules:
+	ethernet@.... {
+		....
+		resets = <&reset HSDK_V1_ETH_RESET>;
+		....
+	};
+
+The index could be found in <dt-bindings/reset/snps,hsdk-v1-reset.h>