nrf: add nrf52840-mdk board

With the help of Chillance on GitHub.

src/machine/board_nrf52840-mdk.go

@@ -0,0 +1,25 @@
+// +build nrf52840_mdk
+
+package machine
+
+const HasLowFrequencyCrystal = true
+
+// LEDs on the nrf52840-mdk (nRF52840 dev board)
+const (
+	LED       = LED_GREEN
+	LED_GREEN = 22
+	LED_RED   = 23
+	LED_BLUE  = 24
+)
+
+// UART pins
+const (
+	UART_TX_PIN = 20
+	UART_RX_PIN = 19
+)
+
+// I2C pins (unused)
+const (
+	SDA_PIN = 0xff
+	SCL_PIN = 0xff
+)