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/stm32h7/src/bin/spi_dma.rs

@@ -10,16 +10,16 @@ use defmt::*;
 use embassy::executor::Executor;
 use embassy::util::Forever;
 use embassy_stm32::peripherals::{DMA1_CH3, DMA1_CH4, SPI3};
-use embassy_stm32::time::U32Ext;
+use embassy_stm32::time::mhz;
 use embassy_stm32::{spi, Config};
 use heapless::String;
 use {defmt_rtt as _, panic_probe as _};
 
 pub fn config() -> Config {
     let mut config = Config::default();
-    config.rcc.sys_ck = Some(400.mhz().into());
-    config.rcc.hclk = Some(200.mhz().into());
-    config.rcc.pll1.q_ck = Some(100.mhz().into());
+    config.rcc.sys_ck = Some(mhz(400));
+    config.rcc.hclk = Some(mhz(200));
+    config.rcc.pll1.q_ck = Some(mhz(100));
     config
 }
 
@@ -50,7 +50,7 @@ fn main() -> ! {
         p.PB4,
         p.DMA1_CH3,
         p.DMA1_CH4,
-        1.mhz(),
+        mhz(1),
         spi::Config::default(),
     );