of: overlay: add per overlay sysfs attributes

* A per overlay can_remove sysfs attribute that reports whether
the overlay can be removed or not due to another overlapping overlay.

* A target sysfs attribute listing the target of each fragment,
in a group named after the name of the fragment.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
[geert: Setup ovinfo[cnt].info for symbols]
[geert: Spelling s/changset/changeset/]
[geert: Rebase on top of commit 39a751a4

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