docs: dts: Add documentation for hi6220 SoC ION node

Hi6220 SoC ION

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Required properties:

- compatible : "hisilicon,hi6220-ion"

- list of the ION heaps

	- heap name : maybe heap_sys_user@0

	- heap id   : id should be unique in the system.

	- heap base : base ddr address of the heap,0 means that

	it is dynamic.

	- heap size : memory size and 0 means it is dynamic.

	- heap type : the heap type of the heap, please also

	see the define in ion.h(drivers/staging/android/uapi/ion.h)

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Example:

	hi6220-ion {

		compatible = "hisilicon,hi6220-ion";

		heap_sys_user@0 {

			heap-name = "sys_user";

			heap-id   = <0x0>;

			heap-base = <0x0>;

			heap-size = <0x0>;

			heap-type = "ion_system";

		};

		heap_sys_contig@0 {

			heap-name = "sys_contig";

			heap-id   = <0x1>;

			heap-base = <0x0>;

			heap-size = <0x0>;

			heap-type = "ion_system_contig";

		};

	};