Enable clock by default for stm32l0

Modify init function to return a Clock instance defined by a per-chip
SystemClock type and use this in macro setup

A proof of concept implementation for STM32 L0 chips.

This allows using embassy::main macros for STM32 devices that have the
clock setup logic.

embassy-stm32/src/rcc/mod.rs

@@ -6,8 +6,10 @@ cfg_if::cfg_if! {
         mod l0;
         pub use l0::*;
     } else {
+        pub type SystemClock = ();
         #[derive(Default)]
         pub struct Config {}
-        pub fn init(_config: Config) {}
+        pub unsafe fn init(_config: Config) -> SystemClock {
+        }
     }
 }