DT: proc: Add runtime overlay interface in /proc

Add a runtime interface to /proc to enable generic device tree overlay
usage.

Two new /proc files are added:

/proc/device-tree-overlay & /proc/device-tree-overlay-status

/proc/device-tree-overlay accepts a stream of a device tree objects and
applies it to the running kernel's device tree.

	$ cat ~/BB-UART2-00A0.dtbo >device-tree-overlay
	overlay_proc_release: Applied #2 overlay segments @0

/proc/device-tree-overlay-status displays the the overlays added using
the /proc interface

	$ cat device-tree-overlay-status
	0: 861 bytes BB-UART2:00A0

The format of the status line is
	<ID>: <SIZE> bytes <part-number>:<version>

<ID> is the id of the overlay
<SIZE> is the size of the overlay in bytes
<part-number>, <version> are (optional) root level properties of the DTBO

You can remove an overlay by echoing the <ID> number of the overlay
precedded with a '-'

So
	$ echo "-0" >device-tree-overlay-status

Removes the overlay.

Note that this seldom works on most platforms since platform_device
removal is something that almost never works without extra patches.

Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>