Enable RTC on STM32WL chips (#1645)

* Add clippy allow to not report if same then branch

* Support enabling RTC clock on STM32WL

* Add clippy allow to not report if same then branch

* Support enabling RTC clock on STM32WL

* Add rtc example for stm32wl

* Address code review feedback

examples/stm32wl/src/bin/rtc.rs

@@ -0,0 +1,43 @@
+#![no_std]
+#![no_main]
+#![feature(type_alias_impl_trait)]
+
+use chrono::{NaiveDate, NaiveDateTime};
+use defmt::*;
+use embassy_executor::Spawner;
+use embassy_stm32::rcc::{self, ClockSrc};
+use embassy_stm32::rtc::{Rtc, RtcConfig};
+use embassy_stm32::Config;
+use embassy_time::{Duration, Timer};
+use {defmt_rtt as _, panic_probe as _};
+
+#[embassy_executor::main]
+async fn main(_spawner: Spawner) {
+    let p = {
+        let mut config = Config::default();
+        config.rcc.mux = ClockSrc::HSE32;
+        config.rcc.rtc_mux = rcc::RtcClockSource::LSE32;
+        config.rcc.enable_rtc_apb = true;
+        embassy_stm32::init(config)
+    };
+    info!("Hello World!");
+
+    let now = NaiveDate::from_ymd_opt(2020, 5, 15)
+        .unwrap()
+        .and_hms_opt(10, 30, 15)
+        .unwrap();
+
+    let mut rtc = Rtc::new(
+        p.RTC,
+        RtcConfig::default().clock_config(embassy_stm32::rtc::RtcClockSource::LSE),
+    );
+    info!("Got RTC! {:?}", now.timestamp());
+
+    rtc.set_datetime(now.into()).expect("datetime not set");
+
+    // In reality the delay would be much longer
+    Timer::after(Duration::from_millis(20000)).await;
+
+    let then: NaiveDateTime = rtc.now().unwrap().into();
+    info!("Got RTC! {:?}", then.timestamp());
+}