ieee802154: Add device tree documentation for CA8210

* CA8210 IEEE 802.15.4 *

Required properties:

	- compatible:           Should be "cascoda,ca8210"

	- reg:                  Controlling chip select

	- spi-max-frequency:    Maximum clock speed, should be *less than*

	                        4000000

	- spi-cpol:             Requires inverted clock polarity

	- reset-gpio:           GPIO attached to reset

	- irq-gpio:             GPIO attached to IRQ

Optional properties:

	- extclock-enable:      Include for the ca8210 to route its 16MHz clock

	                        to an output

	- extclock-freq:        Frequency in Hz of the external clock

	- extclock-gpio:        GPIO of the ca8210 to output the clock on

Example:

	ca8210@0 {

		compatible = "cascoda,ca8210";

		reg = <0>;

		spi-max-frequency = <3000000>;

		spi-cpol;

		reset-gpio = <&gpio1 1 GPIO_ACTIVE_HIGH>;

		irq-gpio = <&gpio1 2 GPIO_ACTIVE_HIGH>;

		extclock-enable;

		extclock-freq = 16000000;

		extclock-gpio = 2;

	};

buffalo	Buffalo, Inc.

calxeda	Calxeda

capella	Capella Microsystems, Inc

cascoda	Cascoda, Ltd.

cavium	Cavium, Inc.

cdns	Cadence Design Systems Inc.

ceva	Ceva, Inc.