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.
