feat(mt8188): initialize systimer

Add systimer to support timer function.

TEST=build pass
BUG=b:236331724

Change-Id: Ibe6b96a162caa8804bebb7ff7de326ebcb2a6daa
diff --git a/plat/mediatek/drivers/timer/mt_timer.h b/plat/mediatek/drivers/timer/mt_timer.h
index 71dbeff..1c08f90 100644
--- a/plat/mediatek/drivers/timer/mt_timer.h
+++ b/plat/mediatek/drivers/timer/mt_timer.h
@@ -30,6 +30,6 @@
 
 void sched_clock_init(uint64_t normal_base, uint64_t atf_base);
 uint64_t sched_clock(void);
-void mt_systimer_init(void);
+int mt_systimer_init(void);
 
 #endif /* MT_TIMER_H */