mcp3008: add implementation for MCP3008 ADC with SPI interface

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

examples/mcp3008/main.go

@@ -0,0 +1,32 @@
+// Connects to a MCP3008 ADC via SPI.
+package main
+
+import (
+	"machine"
+	"time"
+
+	"tinygo.org/x/drivers/mcp3008"
+)
+
+var (
+	spi   = machine.SPI0
+	csPin = machine.D12
+)
+
+func main() {
+	spi.Configure(machine.SPIConfig{
+		Frequency: 4000000,
+		Mode:      3})
+
+	adc := mcp3008.New(spi, csPin)
+	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)
+	}
+}