of: overlay: kobjectify overlay objects

We are going to need the overlays to appear on sysfs with runtime
global properties (like master enable) so turn them into kobjects.

They have to be in sysfs so that people can have information about the
overlays applied in the system, i.e. where their targets are and whether
removal is possible. In a future more attributes can be added
in a backwards compatible manner.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
[geert: Rebase to v4.15-rc1]
[Fengguang Wu: Make overlay_changeset_release() static]
[geert: Rebase on top of commit 39a751a4

 ("of: change overlay apply input data from unflattened to FDT")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>