clocksource/drivers/mediatek: Use GPT as sched clock source

When cpu is in deep idle, arch timer will stop counting. Setup GPT as
sched clock source so it can keep counting in idle.

Signed-off-by: Yingjoe Chen <yingjoe.chen@mediatek.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Matthias Brugger <matthias.bgg@gmail.com>