stm32/h7: Implemented support for the LSI clock source

include/libopencm3/stm32/h7/rcc.h

@@ -778,6 +778,11 @@ void rcc_clock_setup_pll(const struct rcc_pll_config *config);
  */
 void rcc_clock_setup_hsi48(void);
 
+/**
+ * Setup and bring up the LSI.
+ */
+void rcc_clock_setup_lsi(void);
+
 /**
  * Get the clock rate (in Hz) of the specified clock source. There are
  * numerous clock sources and configurations on the H7, so rates for each

lib/stm32/h7/rcc.c

@@ -230,6 +230,13 @@ void rcc_clock_setup_hsi48(void)
 		continue;
 }
 
+void rcc_clock_setup_lsi(void)
+{
+	RCC_CSR |= RCC_CSR_LSION;
+	while (!(RCC_CSR & RCC_CSR_LSIRDY))
+		continue;
+}
+
 uint32_t rcc_get_bus_clk_freq(enum rcc_clock_source source) {
 	uint32_t clksel;
 	switch (source) {