ITE drivers/timer: add disable event timer control

We add disable event timer at the beginning of critical section
for two reason:
1.For K_TICKS_FOREVER case: since no future timer interrupts
are expected or required, so we disable the event timer.
2.Others case: according it81202 spec, when timer enable bit
from 0->1, the timer will reload counts and start countdown.

Signed-off-by: Ruibin Chang <ruibin.chang@ite.com.tw>