clocksource/drivers/timer-mediatek: Add system timer bindings

This patch adds bindings of new "System Timer" on Mediatek SoCs.

Remove RTC clock in the same time because it is not used by
both "General Purpose Timer" and "System Timer" now.

Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>