mrf24j40: add device-tree support

This patch adds devicetree support to mrf24j40 with proper devicetree
compatible strings.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

Documentation/devicetree/bindings/net/ieee802154/mrf24j40.txt

@@ -0,0 +1,20 @@
+* MRF24J40 IEEE 802.15.4 *
+
+Required properties:
+  - compatible:		should be "microchip,mrf24j40", "microchip,mrf24j40ma",
+			or "microchip,mrf24j40mc" depends on your transceiver
+			board
+  - spi-max-frequency:	maximal bus speed, should be set something under or equal
+			10000000
+  - reg:		the chipselect index
+  - interrupts:		the interrupt generated by the device.
+
+Example:
+
+	mrf24j40ma@0 {
+		compatible = "microchip,mrf24j40ma";
+		spi-max-frequency = <8500000>;
+		reg = <0>;
+		interrupts = <19 8>;
+		interrupt-parent = <&gpio3>;
+	};