ieee802154: Add device tree documentation for MCR20A

Signed-off-by: Xue Liu <liuxuenetmail@gmail.com>
Signed-off-by: Stefan Schmidt <stefan@osg.samsung.com>

Documentation/devicetree/bindings/net/ieee802154/mcr20a.txt

+* MCR20A IEEE 802.15.4 *
+
+Required properties:
+  - compatible:		should be "nxp,mcr20a"
+  - spi-max-frequency:	maximal bus speed, should be set to a frequency
+			lower than 9000000 depends sync or async operation mode
+  - reg:		the chipselect index
+  - interrupts:		the interrupt generated by the device. Non high-level
+			can occur deadlocks while handling isr.
+
+Optional properties:
+  - rst_b-gpio:		GPIO spec for the RST_B pin
+
+Example:
+
+	mcr20a@0 {
+		compatible = "nxp,mcr20a";
+		spi-max-frequency = <9000000>;
+		reg = <0>;
+		interrupts = <17 2>;
+		interrupt-parent = <&gpio>;
+		rst_b-gpio = <&gpio 27 1>
+	};