clocksource: clps711x: Add DT bindings documentation

This patch adds DT binding documentation for the Cirrus Logic
CLPS711X-based CPUs clocksource subsystem.
Signed-off-by: NAlexander Shiyan <shc_work@mail.ru>
Acked-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NDaniel Lezcano <daniel.lezcano@linaro.org>

Documentation/devicetree/bindings/timer/cirrus,clps711x-timer.txt

+* Cirrus Logic CLPS711X Timer Counter
+
+Required properties:
+- compatible: Shall contain "cirrus,clps711x-timer".
+- reg       : Address and length of the register set.
+- interrupts: The interrupt number of the timer.
+- clocks    : phandle of timer reference clock.
+
+Note: Each timer should have an alias correctly numbered in "aliases" node.
+
+Example:
+	aliases {
+		timer0 = &timer1;
+		timer1 = &timer2;
+	};
+
+	timer1: timer@80000300 {
+		compatible = "cirrus,ep7312-timer", "cirrus,clps711x-timer";
+		reg = <0x80000300 0x4>;
+		interrupts = <8>;
+		clocks = <&clks 5>;
+	};
+
+	timer2: timer@80000340 {
+		compatible = "cirrus,ep7312-timer", "cirrus,clps711x-timer";
+		reg = <0x80000340 0x4>;
+		interrupts = <9>;
+		clocks = <&clks 6>;
+	};