devicetree: add device tree bindings for cc2520 driver

DT bindings for cc2520 radio driver
Signed-off-by: Varka Bhadram <varkab@cdac.in>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/ieee802154/cc2520.txt

+*CC2520 IEEE 802.15.4 Compatible Radio*
+
+Required properties:
+	- compatible: 		should be "ti,cc2520"
+	- spi-max-frequency:	maximal bus speed (8000000), should be set to 4000000 depends
+				sync or async operation mode
+	- reg:			the chipselect index
+	- pinctrl-0: 		pin control group to be used for this controller.
+	- pinctrl-names: 	must contain a "default" entry.
+	- fifo-gpio:		GPIO spec for the FIFO pin
+	- fifop-gpio:		GPIO spec for the FIFOP pin
+	- sfd-gpio:		GPIO spec for the SFD pin
+	- cca-gpio:		GPIO spec for the CCA pin
+	- vreg-gpio:		GPIO spec for the VREG pin
+	- reset-gpio:		GPIO spec for the RESET pin
+Example:
+	cc2520@0 {
+		compatible = "ti,cc2520";
+		reg = <0>;
+		spi-max-frequency = <4000000>;
+		pinctrl-names = "default";
+		pinctrl-0 = <&cc2520_cape_pins>;
+		fifo-gpio = <&gpio1 18 0>;
+		fifop-gpio = <&gpio1 19 0>;
+		sfd-gpio = <&gpio1 13 0>;
+		cca-gpio = <&gpio1 16 0>;
+		vreg-gpio = <&gpio0 31 0>;
+		reset-gpio = <&gpio1 12 0>;
+	};