clocksource/drivers/timer-mediatek: Add support for system timer

This patch adds a new "System Timer" on the Mediatek SoCs.

The System Timer is introduced as an always-on timer being
clockevent device for tick-broadcasting.

For clock, it is driven by 13 MHz system clock.
The implementation uses the system clock with no clock
source divider.

For interrupt, the clock event timer can be used by all cores.

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