ARM: clocksource: moxart: documentation: Fix device tree bindings document

Fix device tree bindings document with the correct clock name.
Signed-off-by: Jonas Jensen <jonas.jensen@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

Documentation/devicetree/bindings/timer/moxa,moxart-timer.txt

@@ -5,7 +5,7 @@ Required properties:
 - compatible : Should be "moxa,moxart-timer"
 - reg : Should contain registers location and length
 - interrupts : Should contain the timer interrupt number
-- clocks : Should contain phandle for APB clock "clkapb"
+- clocks : Should contain phandle for the MOXA ART core clock "coreclk"
 
 Example:
 
@@ -13,5 +13,5 @@ Example:
 		compatible = "moxa,moxart-timer";
 		reg = <0x98400000 0x42>;
 		interrupts = <19 1>;
-		clocks = <&clkapb>;
+		clocks = <&coreclk>;
 	};