devicetree: bindings: document Broadcom CPU enable method

Broadcom Kona Family CPU Enable Method

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This binding defines the enable method used for starting secondary

CPUs in the following Broadcom SoCs:

  BCM11130, BCM11140, BCM11351, BCM28145, BCM28155, BCM21664

The enable method is specified by defining the following required

properties in the "cpus" device tree node:

  - enable-method = "brcm,bcm11351-cpu-method";

  - secondary-boot-reg = <...>;

The secondary-boot-reg property is a u32 value that specifies the

physical address of the register used to request the ROM holding pen

code release a secondary CPU.  The value written to the register is

formed by encoding the target CPU id into the low bits of the

physical start address it should jump to.

Example:

	cpus {

		#address-cells = <1>;

		#size-cells = <0>;

		enable-method = "brcm,bcm11351-cpu-method";

		secondary-boot-reg = <0x3500417c>;

		cpu0: cpu@0 {

			device_type = "cpu";

			compatible = "arm,cortex-a9";

			reg = <0>;

		};

		cpu1: cpu@1 {

			device_type = "cpu";

			compatible = "arm,cortex-a9";

			reg = <1>;

		};

	};