docs: dts: Add documentation for hi6220 SoC ION node

Documentation for hi6220 SoC ION node
Signed-off-by: Chen Feng <puck.chen@hisilicon.com>
Signed-off-by: Yu Dongbin <yudongbin@hisilicon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/staging/ion/hi6220-ion.txt

+Hi6220 SoC ION
+===================================================================
+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)
+-------------------------------------------------------------------
+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";
+		};
+	};