staging/android/ion: remove useless document file

After commit 9828282e33a0 ("staging: android: ion: Remove old platform
support"), the document about devicetree of ion is no need anymore, so
just remove it.

Signed-off-by: Yisheng Xie <[e-mail]>
Acked-by: Laura Abbott <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

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";
-		};
-	};

MAINTAINERS

···
 M:	Sumit Semwal <[e-mail]>
 L:	[e-mail]
 S:	Supported
-F:	Documentation/devicetree/bindings/staging/ion/
 F:	drivers/staging/android/ion
 F:	drivers/staging/android/uapi/ion.h
 F:	drivers/staging/android/uapi/ion_test.h

drivers/staging/android/ion/devicetree.txt

···
-Ion Memory Manager
-
-Ion is a memory manager that allows for sharing of buffers via dma-buf.
-Ion allows for different types of allocation via an abstraction called
-a 'heap'. A heap represents a specific type of memory. Each heap has
-a different type. There can be multiple instances of the same heap
-type.
-
-Specific heap instances are tied to heap IDs. Heap IDs are not to be specified
-in the devicetree.
-
-Required properties for Ion
-
-- compatible: "linux,ion" PLUS a compatible property for the device
-
-All child nodes of a linux,ion node are interpreted as heaps
-
-required properties for heaps
-
-- compatible: compatible string for a heap type PLUS a compatible property
-for the specific instance of the heap. Current heap types
--- linux,ion-heap-system
--- linux,ion-heap-system-contig
--- linux,ion-heap-carveout
--- linux,ion-heap-chunk
--- linux,ion-heap-dma
--- linux,ion-heap-custom
-
-Optional properties
-- memory-region: A phandle to a memory region. Required for DMA heap type
-(see reserved-memory.txt for details on the reservation)
-
-Example:
-
-	ion {
-		compatbile = "hisilicon,ion", "linux,ion";
-
-		ion-system-heap {
-			compatbile = "hisilicon,system-heap", "linux,ion-heap-system"
-		};
-
-		ion-camera-region {
-			compatible = "hisilicon,camera-heap", "linux,ion-heap-dma"
-			memory-region = <&camera_region>;
-		};
-
-		ion-fb-region {
-			compatbile = "hisilicon,fb-heap", "linux,ion-heap-dma"
-			memory-region = <&fb_region>;
-		};
-	}