Unify counter.h and rtc.h interfaces

[nordic-krch]

Counter.h and rtc.h share almost the same functionality with only minor
differences:

• rtc.h has option for initial value and initial alarm (set by rtc_set_config)
• counter.h has pointer to the user context in the alarm callback

Both interfaces have drawbacks:

• no information about the frequency so alarm is set in ticks which are virtual
• no helper function to convert microseconds or milliseconds to ticks
• in both APIs it is unclear if alarm is one shot or periodic
• interfaces are limiting functionality of peripherals (TIMER, RTC) which usually
  have couple of capture/compare channels. Current interfaces allow usage of single channel
• lack of information about overflow (unless alarm is periodic and is actual overflow)
• lack of possibility to set alarm for absolute value

Counter.h and RTC.h should be unified. Implementation of unified interface can be based on low power RTC-like peripheral or more accurate high frequency general purpose timers.

Topic already mentioned in #5895

Pull Requests, targeting the topic-counters topic branch:

@nordic-krch to coordinate with @anangl once all PRs are posted.

• API + nRF + tests : PR [DNM] [RFC] drivers: counter: API proposal unifying counter and rtc drivers. #8340
• nRF: PR [DNM] Nrf counter shim #8743
• CMSDK: PR Move CMSDK Counters/Timers to unified interface #10025
• QMSI: Move AON, AONPT counter and QMSI RTC to unified interface #9276
• mcux: Introduce mcux driver for new unified counter/rtc interface #11347
• stm32: Move STM32 RTC to new counter API #11373
• i.MX EPIT: i.MX EPIT Timer Driver #11441

[carlescufi]

Related issues: #6498 and #7520

[carlescufi]

Merged into master