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>
diff --git a/include/stm32_rcc.h b/include/stm32_rcc.h
index 063177b..484bc30 100644
--- a/include/stm32_rcc.h
+++ b/include/stm32_rcc.h
@@ -45,6 +45,11 @@
 	STM32F7,
 };
 
+enum apb {
+	APB1,
+	APB2,
+};
+
 struct stm32_rcc_clk {
 	char *drv_name;
 	enum soc_family soc;