drivers: clock: get_pllsrc_frequency for stm32f2/f4/f7

The function get_pllsrc_frequency is added
with possible source HSI or HSE.
With STM32_PLL_P_ENABLED or STM32_PLL_Q_ENABLED,
this function is used by the clock control driver.

Signed-off-by: Francois Ramu <francois.ramu@st.com>

drivers/clock_control/clock_stm32f2_f4_f7.c

@@ -33,6 +33,22 @@ static uint32_t get_pll_source(void)
 	return 0;
 }
 
+/**
+ * @brief get the pll source frequency
+ */
+__unused
+uint32_t get_pllsrc_frequency(void)
+{
+	if (IS_ENABLED(STM32_PLL_SRC_HSI)) {
+		return STM32_HSI_FREQ;
+	} else if (IS_ENABLED(STM32_PLL_SRC_HSE)) {
+		return STM32_HSE_FREQ;
+	}
+
+	__ASSERT(0, "Invalid source");
+	return 0;
+}
+
 /**
  * @brief Set up pll configuration
  */