arm/lc823450: Use clock_systime_ticks() to access system ticks

    Use clock_get_sched_tclock_systime_ticks() to access system ticks
     in order to fix the build error and improve code safety.

Signed-off-by: Chengdong Wang <wangchengdong@lixiang.com>

Author: wangchdo

arch/arm/src/lc823450/lc823450_timer.c

@@ -705,7 +705,7 @@ int up_rtc_gettime(struct timespec *tp)
 
   /* Get the elapsed time */
 
-  elapsed = NSEC_PER_TICK * (uint64_t)g_system_ticks;
+  elapsed = NSEC_PER_TICK * (uint64_t)clock_systime_ticks();
 
   /* Add the timer fraction in nanoseconds */