machine/feather-m0: correct board build tag and add placeholders for I2S interface

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

src/machine/board_feather-m0.go

@@ -1,4 +1,4 @@
-// +build sam,atsamd21,itsybitsy_m0
+// +build sam,atsamd21,feather_m0
 
 package machine
 
@@ -73,3 +73,10 @@ const (
 var (
 	SPI0 = SPI{Bus: sam.SERCOM4_SPI}
 )
+
+// I2S pins
+const (
+	I2S_SCK_PIN = PA10
+	I2S_SD_PIN  = PA08
+	I2S_WS_PIN  = 0xff // TODO: figure out what this is on Feather M0.
+)