add 4x4 keypad driver (#226)

add 4x4 keypad driver

examples/keypad4x4/main.go

@@ -0,0 +1,38 @@
+package main
+
+import (
+	"machine"
+
+	"tinygo.org/x/drivers/keypad4x4"
+)
+
+func main() {
+	mapping := map[uint8]string{
+		1:  "1",
+		2:  "2",
+		3:  "3",
+		4:  "A",
+		5:  "4",
+		6:  "5",
+		7:  "6",
+		8:  "B",
+		9:  "7",
+		10: "8",
+		11: "9",
+		12: "C",
+		13: "*",
+		14: "0",
+		15: "#",
+		16: "D",
+	}
+
+	keypadDevice := keypad4x4.NewDevice(machine.D2, machine.D3, machine.D4, machine.D5, machine.D6, machine.D7, machine.D8, machine.D9)
+	keypadDevice.Configure()
+
+	for {
+		key := keypadDevice.GetKey()
+		if key != keypad4x4.NoKeyPressed {
+			println("Button: ", mapping[key])
+		}
+	}
+}