Re-implemented both us_ticker and lp_ticker to match the new API and specifications.
Details:
* On EFM32GG, EFM32WG, EFM32LG, EFM32HG, EFM32ZG: Use the RTC peripheral to back lp_ticker, and a TIMER to back us_ticker.
* On EFM32PG, EFR32MG, EFM32PG12, EFR32MG12: Use the RTCC peripheral to back lp_ticker (dual-purpose, also used to back RTC), and a TIMER to back us_ticker.
This commit fixes#5840. Fix verified by running mbed_hal-lp_ticker test suite with preloaded RTC counter such that it wrapped in the middle of the suite.
Also removes explicit sleep blocking from the us_ticker implementation, since sleep blocking for us tickers is done at mbed HAL level now. This was causing one of the lp_ticker tests to fail.
Moving the per-board clock configuration (which oscillators are available on the board, their frequencies, and which ones to use) as config options to the target database. This way, they're more easily overridable when third parties start creating boards with EFM32 MCUs
First wave of squashing header files in order to get to a more sensible folder tree.
* Moved header files with 100% similarity to common folder
* Created a CommonPinNames.h header containing the base pin names P[A-F][0-15] and NC
Steven Cooreman