drivers/clocksource/mtk_timer.c: add a somehow working version

This adds a somehow working version of the timer. It seems that the
system stops when "Switched to clocksource timer"
ARM: mediatek: add mtk6589 plattform files
arm: dts: mtk6589: clean up DT files
clocksource: mtk_timer.c: implement multi timer support
ARM: DTS: mt6589-clocks: define some fixed clocks for now
ARM: DTS: mtk6589.dtsi: Add new timer DT node
Documentation: DTS: mtk-timer.txt: use just one register definition