devicetree: add at86rf230 bindings

This patch adds devicetree bindings for the at86rf230 IEEE 802.15.4 SPI
device driver.
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/ieee802154/at86rf230.txt

+* AT86RF230 IEEE 802.15.4 *
+
+Required properties:
+  - compatible:		should be "atmel,at86rf230", "atmel,at86rf231",
+			"atmel,at86rf233" or "atmel,at86rf212"
+  - spi-max-frequency:	maximal bus speed, should be set to 7500000 depends
+			sync or async operation mode
+  - reg:		the chipselect index
+  - interrupts:		the interrupt generated by the device
+
+Optional properties:
+  - reset-gpio:		GPIO spec for the rstn pin
+  - sleep-gpio:		GPIO spec for the slp_tr pin
+
+Example:
+
+	at86rf231@0 {
+		compatible = "atmel,at86rf231";
+		spi-max-frequency = <7500000>;
+		reg = <0>;
+		interrupts = <19 1>;
+		interrupt-parent = <&gpio3>;
+	};