stm32f7: add pwm

src/machine/machine_stm32f7x2.go

@@ -12,6 +12,12 @@ func CPUFrequency() uint32 {
 	return 216000000
 }
 
+// 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 = 54e6  // 54MHz
+const APB2_TIM_FREQ = 216e6 // 216MHz
+
 //---------- UART related code
 
 // Configure the UART.