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/h7/mod.rs

@@ -528,6 +528,6 @@ impl<'d> Rcc<'d> {
     }
 }
 
-pub unsafe fn init(config: Config) {
-    // TODO
-}
+// TODO
+pub type SystemClock = ();
+pub unsafe fn init(_config: Config) -> SystemClock {}