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.
Adds RccPeripheral trait for peripherals implementing clock enable and reset for a given peripheral.
Add macro table generting implementations of RccPeripheral for peripherals with clock set, currently restricted to SPI.
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.
