stm32/system_stm32f0: Enable PWR clock on startup.

To be consistent with how F4/F7/H7/L4 works in system_stm32.c.  The power
control peripheral is needed at least for the RTC.

ports/stm32/system_stm32f0.c

@@ -129,6 +129,9 @@ void SystemInit(void) {
 }
 
 void SystemClock_Config(void) {
+    // Enable power control peripheral
+    __HAL_RCC_PWR_CLK_ENABLE();
+
     // Set flash latency to 1 because SYSCLK > 24MHz
     FLASH->ACR = (FLASH->ACR & ~0x7) | 0x1;