Some periph_rtt implementations do not provide `rtt_set_counter()`. This
adds `periph_rtt_set_counter` as feature to allow testing for its
availability. The feature is provided at CPU level if periph_rtt is
provided by the board for all CPUs implementing `rtt_set_counter()`.
MCG_Lite is used in many KL parts and is a less advanced clock generator
than the full MCG used in the K series. This change lets the MCG_Lite
and MCG share the same user facing API, with some configuration
differences.