clocksource: add DT bindings for Cortina Gemini

This adds device tree bindings for the Cortina Systems Gemini
timer block used in these SoCs.

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>
Cc: devicetree@vger.kernel.org
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NDaniel Lezcano <daniel.lezcano@linaro.org>

Documentation/devicetree/bindings/timer/cortina,gemini-timer.txt

+Cortina Systems Gemini timer
+
+This timer is embedded in the Cortina Systems Gemini SoCs.
+
+Required properties:
+
+- compatible : Must be "cortina,gemini-timer"
+- reg : Should contain registers location and length
+- interrupts : Should contain the three timer interrupts with
+  flags for rising edge
+- syscon : a phandle to the global Gemini system controller
+
+Example:
+
+timer@43000000 {
+	compatible = "cortina,gemini-timer";
+	reg = <0x43000000 0x1000>;
+	interrupts = <14 IRQ_TYPE_EDGE_RISING>, /* Timer 1 */
+		   <15 IRQ_TYPE_EDGE_RISING>, /* Timer 2 */
+		   <16 IRQ_TYPE_EDGE_RISING>; /* Timer 3 */
+	syscon = <&syscon>;
+};