Document: DT: Add bindings for mediatek MT6580 SoC Platform

This adds a DT binding documentation for the MT6580 SoC from Mediatek.

Signed-off-by: Mars Cheng <mars.cheng@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>

Documentation/devicetree/bindings/timer/mediatek,mtk-timer.txt

@@ -2,7 +2,11 @@ Mediatek MT6577, MT6572 and MT6589 Timers
 ---------------------------------------
 
 Required properties:
-- compatible: Should be "mediatek,mt6577-timer"
+- compatible should contain:
+	* "mediatek,mt6589-timer" for MT6589 compatible timers
+	* "mediatek,mt6580-timer" for MT6580 compatible timers
+	* "mediatek,mt6577-timer" for all compatible timers (MT6589, MT6580,
+		MT6577)
 - reg: Should contain location and length for timers register.
 - clocks: Clocks driving the timer hardware. This list should include two
 	clocks. The order is system clock and as second clock the RTC clock.