stm32: add pwm for f4 series

src/machine/machine_stm32f405.go

@@ -13,6 +13,12 @@ func CPUFrequency() uint32 {
 	return 168000000
 }
 
+// Internal use: configured speed of the APB1 and APB2 timers, this should be kept
+// in sync with any changes to runtime package which configures the oscillators
+// and clock frequencies
+const APB1_TIM_FREQ = 42000000 * 2
+const APB2_TIM_FREQ = 84000000 * 2
+
 // Alternative peripheral pin functions
 const (
 	AF0_SYSTEM                = 0