ARM: mediatek: enable gpt6 on boot up to make arch timer working

We enable GTP6 which ungates the arch timer clock.
In the future this should be done in the bootloader.

Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Yingjoe Chen <yingjoe.chen@mediatek.com>