stm32: Fix watchdog division by zero for 256 prescaler, add watchdog example for H7

examples/stm32h7/src/bin/wdg.rs

@@ -0,0 +1,24 @@
+#![no_std]
+#![no_main]
+#![feature(type_alias_impl_trait)]
+
+use defmt::*;
+use embassy_executor::Spawner;
+use embassy_stm32::wdg::IndependentWatchdog;
+use embassy_time::{Duration, Timer};
+use {defmt_rtt as _, panic_probe as _};
+
+#[embassy_executor::main]
+async fn main(_spawner: Spawner) {
+    let p = embassy_stm32::init(Default::default());
+    info!("Hello World!");
+
+    let mut wdg = IndependentWatchdog::new(p.IWDG1, 20_000_000);
+
+    unsafe { wdg.unleash() };
+
+    loop {
+        Timer::after(Duration::from_secs(1)).await;
+        unsafe { wdg.pet() };
+    }
+}