stm32/f0: RTC clock helpers

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>

include/libopencm3/stm32/f0/rcc.h

@@ -509,6 +509,9 @@ void rcc_css_int_clear(void);
 int rcc_css_int_flag(void);
 void rcc_set_sysclk_source(enum rcc_osc clk);
 void rcc_set_usbclk_source(enum rcc_osc clk);
+void rcc_set_rtc_clock_source(enum rcc_osc clk);
+void rcc_enable_rtc_clock(void);
+void rcc_disable_rtc_clock(void);
 void rcc_set_pll_multiplication_factor(uint32_t mul);
 void rcc_set_ppre(uint32_t ppre);
 void rcc_set_hpre(uint32_t hpre);