nrf: add nrf52840-mdk board

With the help of Chillance on GitHub.

src/machine/machine_nrf52840.go

@@ -0,0 +1,31 @@
+// +build nrf52840
+
+package machine
+
+import (
+	"device/nrf"
+)
+
+// Get peripheral and pin number for this GPIO pin.
+func (p GPIO) getPortPin() (*nrf.GPIO_Type, uint8) {
+	if p.Pin >= 32 {
+		return nrf.P1, p.Pin - 32
+	} else {
+		return nrf.P0, p.Pin
+	}
+}
+
+func (uart UART) setPins(tx, rx uint32) {
+	nrf.UART0.PSEL.TXD = nrf.RegValue(tx)
+	nrf.UART0.PSEL.RXD = nrf.RegValue(rx)
+}
+
+//go:export UARTE0_UART0_IRQHandler
+func handleUART0() {
+	UART0.handleInterrupt()
+}
+
+func (i2c I2C) setPins(scl, sda uint8) {
+	i2c.Bus.PSEL.SCL = nrf.RegValue(scl)
+	i2c.Bus.PSEL.SDA = nrf.RegValue(sda)
+}