timer: mtk_timer: initialize the timer before use

The timer being used by this driver may have already been used by first
stage bootloader (e.g. ATF/preloader), and it's settings may differ from
what this driver is going to use.

This may cause issues, such as inaccurate timer frequency due to
incorrect clock divider.

This patch adds the initialization code to avoid them.

Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>