[Nuvoton] Power down RTC access from CPU domain in rtc_free

After rtc_free, RTC gets inaccessible from CPU domain but keeps counting.

Author: ccli8

targets/TARGET_NUVOTON/TARGET_M451/rtc_api.c

@@ -100,7 +100,7 @@ void rtc_init(void)
 
 void rtc_free(void)
 {
-    // N/A
+    CLK_DisableModuleClock(rtc_modinit.clkidx);
 }
 
 int rtc_isenabled(void)

targets/TARGET_NUVOTON/TARGET_M480/rtc_api.c

@@ -100,7 +100,7 @@ void rtc_init(void)
 
 void rtc_free(void)
 {
-    // N/A
+    CLK_DisableModuleClock(rtc_modinit.clkidx);
 }
 
 int rtc_isenabled(void)
@@ -114,6 +114,7 @@ int rtc_isenabled(void)
     // NOTE: Check RTC Init Active flag to support crossing reset cycle.
     return !! (RTC->INIT & RTC_INIT_ACTIVE_Msk);
 }
+
 time_t rtc_read(void)
 {
     /* NOTE: After boot, RTC time registers are not synced immediately, about 1 sec latency.

targets/TARGET_NUVOTON/TARGET_NANO100/rtc_api.c

@@ -100,7 +100,7 @@ void rtc_init(void)
 
 void rtc_free(void)
 {
-    // N/A
+    CLK_DisableModuleClock(rtc_modinit.clkidx);
 }
 
 int rtc_isenabled(void)

targets/TARGET_NUVOTON/TARGET_NUC472/rtc_api.c

@@ -100,7 +100,7 @@ void rtc_init(void)
 
 void rtc_free(void)
 {
-    // N/A
+    CLK_DisableModuleClock(rtc_modinit.clkidx);
 }
 
 int rtc_isenabled(void)