embassy-stm32: Simplify time

- Remove unused `MilliSeconds`, `MicroSeconds`, and `NanoSeconds` types
- Remove `Bps`, `KiloHertz`, and `MegaHertz` types that were only used
for converting to `Hertz`
- Replace all instances of `impl Into<Hertz>` with `Hertz`
- Add `hz`, `khz`, and `mhz` methods to `Hertz`, as well as
free function shortcuts
- Remove `U32Ext` extension trait

embassy-stm32/src/sdmmc/mod.rs

@@ -261,7 +261,7 @@ impl<'d, T: Instance, P: Pins<T>> Sdmmc<'d, T, P, NoDma> {
 
 impl<'d, T: Instance, P: Pins<T>, Dma: SdmmcDma<T>> Sdmmc<'d, T, P, Dma> {
     #[inline(always)]
-    pub async fn init_card(&mut self, freq: impl Into<Hertz>) -> Result<(), Error> {
+    pub async fn init_card(&mut self, freq: Hertz) -> Result<(), Error> {
         let inner = T::inner();
         let freq = freq.into();