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

examples/stm32f7/src/bin/eth.rs

@@ -12,7 +12,7 @@ use embassy_stm32::eth::generic_smi::GenericSMI;
 use embassy_stm32::eth::{Ethernet, State};
 use embassy_stm32::peripherals::ETH;
 use embassy_stm32::rng::Rng;
-use embassy_stm32::time::U32Ext;
+use embassy_stm32::time::mhz;
 use embassy_stm32::{interrupt, Config, Peripherals};
 use embedded_io::asynch::Write;
 use rand_core::RngCore;
@@ -35,7 +35,7 @@ async fn net_task(stack: &'static Stack<Device>) -> ! {
 
 fn config() -> Config {
     let mut config = Config::default();
-    config.rcc.sys_ck = Some(200.mhz().into());
+    config.rcc.sys_ck = Some(mhz(200));
     config
 }