drivers: counter: rtc: Deprecating legacy counter and rtc API

Deprecating counter legacy API and renaming new counter_set_alarm
to coutner_set_ch_alarm. Deprecating rtc API and modifying rtc
API to call new counter API (compatibility layer).

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>