clk: clps711x: Add DT bindings documentation

This patch adds DT binding documentation for the Cirrus Logic
CLPS711X-based CPUs clock subsystem.
Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mike Turquette <mturquette@linaro.org>

Documentation/devicetree/bindings/clock/clps711x-clock.txt

+* Clock bindings for the Cirrus Logic CLPS711X CPUs
+
+Required properties:
+- compatible       : Shall contain "cirrus,clps711x-clk".
+- reg              : Address of the internal register set.
+- startup-frequency: Factory set CPU startup frequency in HZ.
+- #clock-cells     : Should be <1>.
+
+The clock consumer should specify the desired clock by having the clock
+ID in its "clocks" phandle cell. See include/dt-bindings/clock/clps711x-clock.h
+for the full list of CLPS711X clock IDs.
+
+Example:
+	clks: clks@80000000 {
+		#clock-cells = <1>;
+		compatible = "cirrus,ep7312-clk", "cirrus,clps711x-clk";
+		reg = <0x80000000 0xc000>;
+		startup-frequency = <73728000>;
+	};