reset: img: Add pistachio reset controller binding document

Add binding document for the Pistachio SoC reset controller
Signed-off-by: Damien Horsley <Damien.Horsley@imgtec.com>
Signed-off-by: James Hartley <james.hartley@imgtec.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

Documentation/devicetree/bindings/reset/img,pistachio-reset.txt

+Pistachio Reset Controller
+=============================================================================
+
+This binding describes a reset controller device that is used to enable and
+disable individual IP blocks within the Pistachio SoC using "soft reset"
+control bits found in the Pistachio SoC top level registers.
+
+The actual action taken when soft reset is asserted is hardware dependent.
+However, when asserted it may not be possible to access the hardware's
+registers, and following an assert/deassert sequence the hardware's previous
+state may no longer be valid.
+
+Please refer to Documentation/devicetree/bindings/reset/reset.txt
+for common reset controller binding usage.
+
+Required properties:
+
+- compatible: Contains "img,pistachio-reset"
+
+- #reset-cells: Contains 1
+
+Example:
+
+	cr_periph: clk@18148000 {
+		compatible = "img,pistachio-cr-periph", "syscon", "simple-mfd";
+		reg = <0x18148000 0x1000>;
+		clocks = <&clk_periph PERIPH_CLK_SYS>;
+		clock-names = "sys";
+		#clock-cells = <1>;
+
+		pistachio_reset: reset-controller {
+			compatible = "img,pistachio-reset";
+			#reset-cells = <1>;
+		};
+	};
+
+Specifying reset control of devices
+=======================================
+
+Device nodes should specify the reset channel required in their "resets"
+property, containing a phandle to the pistachio reset device node and an
+index specifying which reset to use, as described in
+Documentation/devicetree/bindings/reset/reset.txt.
+
+Example:
+
+	spdif_out: spdif-out@18100d00 {
+		...
+		resets = <&pistachio_reset PISTACHIO_RESET_SPDIF_OUT>;
+		reset-names = "rst";
+		...
+	};
+
+Macro definitions for the supported resets can be found in:
+include/dt-bindings/reset/pistachio-resets.h