stm32f3: flash: add prefetch helpers

Should be added to f2/f4 as well, but the bit definitions are different.

lib/stm32/f3/flash.c

@@ -58,5 +58,26 @@ void flash_clear_status_flags(void)
 	flash_clear_eop_flag();
 	flash_clear_bsy_flag();
 }
+
+/**
+ * Enable the FLASH Prefetch Buffer
+ * This buffer is used for instruction fetches and is enabled by default after
+ * reset.
+ */
+void flash_prefetch_enable(void)
+{
+	FLASH_ACR |= FLASH_ACR_PRFTBE;
+}
+
+/**
+ * Disable the FLASH Prefetch Buffer
+ * This buffer is used for instruction fetches and is enabled by default after
+ * reset.
+ */
+void flash_prefetch_disable(void)
+{
+	FLASH_ACR &= ~FLASH_ACR_PRFTBE;
+}
+
 /**@}*/