Fix unlocking the backup domain when enabling LSE

Set PWREN bit to enable the power interface clock before enabling access to the backup domain.

embassy-stm32/src/rcc/l4.rs

@@ -412,6 +412,7 @@ pub(crate) unsafe fn init(config: Config) {
     match config.rtc_mux {
         RtcClockSource::LSE32 => {
             // 1. Unlock the backup domain
+            RCC.apb1enr1().modify(|w| w.set_pwren(true));
             PWR.cr1().modify(|w| w.set_dbp(true));
 
             // 2. Setup the LSE