ieee802154: Add device tree documentation for MCR20A

* 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>

	};