Provide a way for a peripheral to query its clock frequency

Currently this looks up the frequency in the global singleton that must
be initialized by the per-chip RCC implementation. At present, this is
only done for the L0 family of chips.

embassy-stm32/src/spi/v2.rs

@@ -37,7 +37,6 @@ pub struct Spi<'d, T: Instance> {
 
 impl<'d, T: Instance> Spi<'d, T> {
     pub fn new<F>(
-        pclk: Hertz,
         _peri: impl Unborrow<Target = T> + 'd,
         sck: impl Unborrow<Target = impl SckPin<T>>,
         mosi: impl Unborrow<Target = impl MosiPin<T>>,
@@ -60,6 +59,7 @@ impl<'d, T: Instance> Spi<'d, T> {
         let mosi = mosi.degrade();
         let miso = miso.degrade();
 
+        let pclk = T::frequency();
         let br = Self::compute_baud_rate(pclk, freq.into());
 
         unsafe {