ARM: mediatek: Add sysirq device node to mt6592 dtsi

Add sysirq node to mt6592.dtsi and also correct timer interrupt flag.
The old setting works because boot loader already set it.
With a sysirq device node, the timer interrupt can use a correct value.

Signed-off-by: Howard Chen <howard.chen@linaro.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>