Added getPrescaler( ); setPrescaler( ); for calibrating STM32F1xx during runtime.

[HamidSaffari]

Added getPrescaler(uint32_t *prescaler); and setPrescaler(uint32_t prescaler); for calibrating STM32F1xx during runtime.

[FRASTM]

The stm32F1 has a different prescaler setting from other stm32 devices. It is purely asynchronous 8+16 bit factor, hold on a 32bit value.
Setting the prescaler is writing the (write-only) PRL registers
Reading the prescaler (on the Read-only reg DIV).

That's why (I guess) new API is proposed for stm32F1 serie using a single 32bit factor.

Another proposal is to keep the existing STM32RTC::setPrediv and STM32RTC::getPrediv
giving a 32bit value (as the 1st param and ignoring the second) in case of stm32F1

[fpistm]

Thanks @HamidSaffari for your contribution and sorry for the delay to handle this PR.
I close this PR as it is superseded by #56.