[Nuvoton] Fix NUC472 hard fault in SMCC tests

Author: cyliangtw

targets/TARGET_NUVOTON/TARGET_NUC472/device/StdDriver/nuc472_clk.c

@@ -75,6 +75,8 @@ void CLK_PowerDown(void)
     SCB->SCR = SCB_SCR_SLEEPDEEP_Msk;
     CLK->PWRCTL |= (CLK_PWRCTL_PDEN_Msk | CLK_PWRCTL_PDWKDLY_Msk );
     __WFI();
+    __DSB();
+    __ISB();
 }
 
 /**
@@ -92,6 +94,8 @@ void CLK_Idle(void)
 
     /* Chip enter idle mode after CPU run WFI instruction */
     __WFI();
+    __DSB();
+    __ISB();
 }