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

1. describe compatible variable "Must be" instead of "Should be".
2. change description so it's from the point of view of the device
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

@@ -2,10 +2,10 @@ MOXA ART timer
 
 Required properties:
 
-- compatible : Should be "moxa,moxart-timer"
+- compatible : Must be "moxa,moxart-timer"
 - reg : Should contain registers location and length
 - interrupts : Should contain the timer interrupt number
-- clocks : Should contain phandle for the MOXA ART core clock "coreclk"
+- clocks : Should contain phandle for the clock that drives the counter
 
 Example: