machine: refactor PWM support

This commit refactors PWM support in the machine package to be more
flexible. The new API can be used to produce tones at a specific
frequency and control servos in a portable way, by abstracting over
counter widths and prescalers.

src/machine/pwm.go

@@ -0,0 +1,21 @@
+package machine
+
+import "errors"
+
+var (
+	ErrPWMPeriodTooLong = errors.New("pwm: period too long")
+)
+
+// PWMConfig allows setting some configuration while configuring a PWM
+// peripheral. A zero PWMConfig is ready to use for simple applications such as
+// dimming LEDs.
+type PWMConfig struct {
+	// PWM period in nanosecond. Leaving this zero will pick a reasonable period
+	// value for use with LEDs.
+	// If you want to configure a frequency instead of a period, you can use the
+	// following formula to calculate a period from a frequency:
+	//
+	//     period = 1e9 / frequency
+	//
+	Period uint64
+}