Add three more RTC clock helper functions:
- rcc_set_rtc_clock_source
RTC on stm32/f0 can be clocked from the following three
sources: LSI, LSE (32.768Hz), HSE/32.
- rcc_enable_rtc_clock
- rcc_disable_rtc_clock
enable/disable clocking RTC module using selected clock source
Signed-off-by: Sergey Matyukevich <geomatsi@gmail.com>