pcf8591: add ADC only implementation for I2C ADC/DAC (#690)

Signed-off-by: deadprogram <ron@hybridgroup.com>

examples/pcf8591/main.go

@@ -0,0 +1,28 @@
+// Connects to a pcf8591 ADC via I2C.
+package main
+
+import (
+	"machine"
+	"time"
+
+	"tinygo.org/x/drivers/pcf8591"
+)
+
+var (
+	i2c = machine.I2C0
+)
+
+func main() {
+	i2c.Configure(machine.I2CConfig{})
+	adc := pcf8591.New(i2c)
+	adc.Configure()
+
+	// get "CH0" aka "machine.ADC" interface to channel 0 from ADC.
+	p := adc.CH0
+
+	for {
+		val := p.Get()
+		println(val)
+		time.Sleep(50 * time.Millisecond)
+	}
+}