rp2040: add machine.ReadTemperature

Replace ADCChannel.ReadTemperature() with a simple ReadTemperature
function.

Not all chips will have a temperature sensor that is read by sampling an
ADC channel. The replacement ReadTemperature is simpler and more generic
to other chip families.

This breaks chips that were relying on the previous ReadTemperature
method. I hope it won't break a lot of existing code. If it does, a
fallback can be added.

src/examples/temp/temp.go

@@ -0,0 +1,23 @@
+// Read the internal temperature sensor of the chip.
+
+package main
+
+import (
+	"fmt"
+	"machine"
+	"time"
+)
+
+type celsius float32
+
+func (c celsius) String() string {
+	return fmt.Sprintf("%4.1f℃", c)
+}
+
+func main() {
+	for {
+		temp := celsius(float32(machine.ReadTemperature()) / 1000)
+		println("temperature:", temp.String())
+		time.Sleep(time.Second)
+	}
+}