STM32L0, L4, F7 and few F4 chip are supporting LPTIM feature.
We propose to allow user to use LPTIM for MBED LowPowerTimer API instead of using RTC wakeup timers.
By default, all targets that are supporting this feature have been configured.
Add RSF synchro during init
Set a better WakeUp clock for long wake up period in order to stay in sleep mode
Use rtc_isenabled function before init as rtc_init is called at each set_time call