Documentation: dt: kona-timer: Add clocks property

The frequency for the Kona timer can either be specified through the
device tree or determined by checking the rate of the clock specified
in the device tree.  Update the documentation to reflect both ways.
Signed-off-by: Tim Kryger <tim.kryger@linaro.org>
Reviewed-by: Matt Porter <matt.porter@linaro.org>
Signed-off-by: Christian Daudt <bcm@fixthebug.org>

Documentation/devicetree/bindings/arm/bcm/kona-timer.txt

@@ -8,13 +8,18 @@ Required properties:
 - DEPRECATED: compatible : "bcm,kona-timer"
 - reg : Register range for the timer
 - interrupts : interrupt for the timer
+- clocks: phandle + clock specifier pair of the external clock
 - clock-frequency: frequency that the clock operates
 
+Only one of clocks or clock-frequency should be specified.
+
+Refer to clocks/clock-bindings.txt for generic clock consumer properties.
+
 Example:
 	timer@35006000 {
 		compatible = "brcm,kona-timer";
 		reg = <0x35006000 0x1000>;
 		interrupts = <0x0 7 0x4>;
-		clock-frequency = <32768>;
+		clocks = <&hub_timer_clk>;
 	};