Counter updates for 6.9

Three key updates of note herein:
  - Introduction of the COUNTER_COMP_FREQUENCY() macro to simplify
    creation of "frequency" Counter extensions
  - Three additional Signals (Clock, Channel 3, and Channel 4) are
    supported for the stm32-timer-cnt
  - Counter events support added for the stm32-timer-cnt

There are also some miscellaneous cleanups and improvements, such as
constifying Counter structures and resolving a kernel-doc description
warning.
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