stm32/rtc: enable lse in examples.

examples/stm32wl/src/bin/rtc.rs

@@ -7,6 +7,7 @@ use defmt::*;
 use embassy_executor::Spawner;
 use embassy_stm32::rcc::ClockSrc;
 use embassy_stm32::rtc::{Rtc, RtcClockSource, RtcConfig};
+use embassy_stm32::time::Hertz;
 use embassy_stm32::Config;
 use embassy_time::{Duration, Timer};
 use {defmt_rtt as _, panic_probe as _};
@@ -16,6 +17,7 @@ async fn main(_spawner: Spawner) {
     let p = {
         let mut config = Config::default();
         config.rcc.mux = ClockSrc::HSE32;
+        config.rcc.lse = Some(Hertz(32_768));
         config.rcc.rtc_mux = RtcClockSource::LSE;
         embassy_stm32::init(config)
     };