clk: dts: document pxa clock binding

Document the device-tree binding of Marvell PXA based SoCs.
PXA clocks are mostly fixed rate and fixed ratio clocks derived from an
external oscillator, and gated by a register set (CKEN or CKEN*).
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Mike Turquette <mturquette@linaro.org>

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

+* Clock bindings for Marvell PXA chips
+
+Required properties:
+- compatible: Should be "marvell,pxa-clocks"
+- #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/.../pxa-clock.h).
+
+Examples:
+
+pxa2xx_clks: pxa2xx_clks@41300004 {
+        compatible = "marvell,pxa-clocks";
+        #clock-cells = <1>;
+        status = "okay";
+};