sam: add support for Atmel SAMD21 based ItsyBitsy M0

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

src/machine/board_itsybitsy-m0.go

@@ -0,0 +1,31 @@
+// +build sam,atsamd21g18a,itsybitsy_m0
+
+package machine
+
+// GPIO Pins
+const (
+	D0  = 11 // RX: SERCOM0/PAD[3]
+	D1  = 10 // TX: SERCOM0/PAD[2]
+	D2  = 14
+	D3  = 9
+	D4  = 8
+	D5  = 15
+	D6  = 20
+	D7  = 21
+	D8  = 6
+	D9  = 7
+	D10 = 18
+	D11 = 16
+	D12 = 19
+	D13 = 17
+)
+
+const (
+	LED = D13
+)
+
+// UART pins
+const (
+	UART_TX_PIN = 0
+	UART_RX_PIN = 0
+)