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.

* Moves the tim2-specific code into macro which always uses TIM2
* For peripherals without clock specified, attempt to locate enable and
  reset registers in the RCC block matching the peripheral name. This
  could be useful for peripherals where deducing the clock name might
  not be feasible, but it remains to be tested with more chip families
  to see if it is sufficiently accurate.

Add ability to enable the hsi48 clock. Code modified from the STM32L0XX
hal

Add temporary start_tim2() fn to Clock to assist macro in starting
embassy clock