ANDROID: GKI: enable CONFIG_MTK_TIMER

It's needed by MTK board for reasons below:
After working with built-in mtk timer(timer-mediatek) as
tick-broadcast devices for few weeks, we found some issues caused
by linux timer framework:
1. tick broadcast installed by insmod cannot switch to oneshot
   mode correctly (BUG: 161822795)
2. rcu warning will be shown if we force to re-enabled tick-broadcast
for each cpu when new tick broadcast device is added by insmod
(timer-mediatek.c in our case)

Bug: 161675989
Change-Id: If3056df11d8256f2af4a6928820faed9f690b3db
Signed-off-by: Freddy Hsin <freddy.hsin@mediatek.com>
Signed-off-by: Chun-Hung Wu <chun-hung.wu@mediatek.com>