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

Fix device tree bindings document with the correct clock name.

Signed-off-by: Jonas Jensen <[e-mail]>
Signed-off-by: Daniel Lezcano <[e-mail]>

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

···
 - 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:
 
···
 		compatible = "moxa,moxart-timer";
 		reg = <0x98400000 0x42>;
 		interrupts = <19 1>;
-		clocks = <&clkapb>;
+		clocks = <&coreclk>;
 	};