power: reset: Add Gemini poweroff DT bindings

This adds device tree bindings to the power management controller
in the Gemini SoC.

Cc: devicetree@vger.kernel.org
Cc: Janos Laube <janos.dev@gmail.com>
Cc: Paulius Zaleckas <paulius.zaleckas@gmail.com>
Cc: Hans Ulli Kroll <ulli.kroll@googlemail.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>

Documentation/devicetree/bindings/power/reset/gemini-poweroff.txt

+* Device-Tree bindings for Cortina Systems Gemini Poweroff
+
+This is a special IP block in the Cortina Gemini SoC that only
+deals with different ways to power the system down.
+
+Required properties:
+- compatible: should be "cortina,gemini-power-controller"
+- reg: should contain the physical memory base and size
+- interrupts: should contain the power management interrupt
+
+Example:
+
+power-controller@4b000000 {
+	compatible = "cortina,gemini-power-controller";
+	reg = <0x4b000000 0x100>;
+	interrupts = <26 IRQ_TYPE_EDGE_FALLING>;
+};