pca10056: support this nrf52840-based board

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

src/machine/board_pca10056.go

@@ -0,0 +1,35 @@
+// +build pca10056
+
+package machine
+
+const HasLowFrequencyCrystal = true
+
+// LEDs on the pca10056
+const (
+	LED  = LED1
+	LED1 = 13
+	LED2 = 14
+	LED3 = 15
+	LED4 = 16
+)
+
+// Buttons on the pca10056
+const (
+	BUTTON  = BUTTON1
+	BUTTON1 = 11
+	BUTTON2 = 12
+	BUTTON3 = 24
+	BUTTON4 = 25
+)
+
+// UART pins
+const (
+	UART_TX_PIN = 6
+	UART_RX_PIN = 8
+)
+
+// I2C pins
+const (
+	SDA_PIN = 26
+	SCL_PIN = 27
+)