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>