Documentation: dt: Add Pistachio SoC general purpose timer binding document

Add a device-tree binding document for the clocksource driver provided
by Pistachio SoC general purpose timers.
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Reviewed-by: Andrew Bresticker <abrestic@chromium.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: devicetree@vger.kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: James Hartley <James.Hartley@imgtec.com>
Cc: Govindraj Raja <Govindraj.Raja@imgtec.com>
Cc: Damien Horsley <Damien.Horsley@imgtec.com>
Cc: James Hogan <James.Hogan@imgtec.com>
Cc: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Patchwork: https://patchwork.linux-mips.org/patch/10783/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

Documentation/devicetree/bindings/timer/img,pistachio-gptimer.txt

+* Pistachio general-purpose timer based clocksource
+
+Required properties:
+ - compatible: "img,pistachio-gptimer".
+ - reg: Address range of the timer registers.
+ - interrupts: An interrupt for each of the four timers
+ - clocks: Should contain a clock specifier for each entry in clock-names
+ - clock-names: Should contain the following entries:
+                "sys", interface clock
+                "slow", slow counter clock
+                "fast", fast counter clock
+ - img,cr-periph: Must contain a phandle to the peripheral control
+		  syscon node.
+
+Example:
+	timer: timer@18102000 {
+		compatible = "img,pistachio-gptimer";
+		reg = <0x18102000 0x100>;
+		interrupts = <GIC_SHARED 60 IRQ_TYPE_LEVEL_HIGH>,
+			     <GIC_SHARED 61 IRQ_TYPE_LEVEL_HIGH>,
+			     <GIC_SHARED 62 IRQ_TYPE_LEVEL_HIGH>,
+			     <GIC_SHARED 63 IRQ_TYPE_LEVEL_HIGH>;
+		clocks = <&clk_periph PERIPH_CLK_COUNTER_FAST>,
+		         <&clk_periph PERIPH_CLK_COUNTER_SLOW>,
+			 <&cr_periph SYS_CLK_TIMER>;
+		clock-names = "fast", "slow", "sys";
+		img,cr-periph = <&cr_periph>;
+	};