Re-adds embassy macros for stm32

* Hook RCC config into chip config and use chip-specific RCC init
  function
* RTC/clock setup is ignored for now

embassy-stm32/src/rcc/mod.rs

@@ -1,4 +1,13 @@
-#[cfg(feature = "_stm32h7")]
-mod h7;
-#[cfg(feature = "_stm32h7")]
-pub use h7::*;
+cfg_if::cfg_if! {
+    if #[cfg(feature = "_stm32h7")] {
+        mod h7;
+        pub use h7::*;
+    } else if #[cfg(feature = "_stm32l0")] {
+        mod l0;
+        pub use l0::*;
+    } else {
+        #[derive(Default)]
+        pub struct Config {}
+        pub fn init(_config: Config) {}
+    }
+}