clk: clk_stm32f: Fix stm32_clk_get_rate() for timer

For timer clock, an additionnal prescaler is used which was
not taken into account previously.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>

include/stm32_rcc.h

@@ -45,6 +45,11 @@ enum soc_family {
 	STM32F7,
 };
 
+enum apb {
+	APB1,
+	APB2,
+};
+
 struct stm32_rcc_clk {
 	char *drv_name;
 	enum soc_family soc;