reset: sunxi: document sunxi's reset controllers bindings

Add DT bindings documentation for sunxi's reset controllers.
Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>

Documentation/devicetree/bindings/reset/allwinner,sunxi-clock-reset.txt

+Allwinner sunxi Peripheral 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:
+  "allwinner,sun6i-a31-ahb1-reset"
+  "allwinner,sun6i-a31-clock-reset"
+- reg: should be register base and length as documented in the
+  datasheet
+- #reset-cells: 1, see below
+
+example:
+
+ahb1_rst: reset@01c202c0 {
+	#reset-cells = <1>;
+	compatible = "allwinner,sun6i-a31-ahb1-reset";
+	reg = <0x01c202c0 0xc>;
+};