If us_ticker/lp_ticker is scheduled and then the interrupt is disabled, the originally scheduled
interrupt may still become pending. If this occurs, then an interrupt will fire twice on the next
call to us_ticker_set_interrupt/lp_ticker_set_interrupt - once immediately and then a second time
at the appropriate time.
This patch prevents the first interrupt by clearing interrupts in
us_ticker_set_interrupt/lp_ticker_set_interrupt before calling NVIC_EnableIRQ.
Originally, we use 2 H/W timers for us_ticker/lp_ticker, one for counting and the other for alarm.
With H/W timer running in continuous mode, we could use just one H/W timer for counting/alarm simultaneously.
The rework includes the following:
1. Remove ticker overflow handling because upper layer (mbed_ticker_api.c) has done with it.
This makes us_ticker/lp_ticker implementation more succinct and avoids potential error.
2. Refine timer register access with low-power clock source
Old lp_ticker handles past event, but it has a bug with premature go-off.
The bug can re-produce on mbed-os-tests-mbed_drivers-lp_timeout/mbed-os-tests-mbed_hal-lp_us_tickers (mbed-os commit: 9c1fd48529).
Because upper layer (mbed-os/hal/mbed_ticker_api.c) has handled past event, this code can be removed from lp_ticker.
The similar fix also applies to us_ticker.
ccli8