OF: DT-Overlay configfs interface (v7)

commit  ba900332d141bf2d1d3b6aa7da8f01fe4b9fb256 from
https://github.com/altera-opensource/linux-socfpga.git

Add a runtime interface to using configfs for generic device tree overlay
usage. With it its possible to use device tree overlays without having
to use a per-platform overlay manager.

Please see Documentation/devicetree/configfs-overlays.txt for more info.

Changes since v6:
- Default groups properties API changed.

Changes since v5:
- New style configfs.

Changes since v4:
- Loading fix for multiple overlays as found out by
  Geert Uytterhoeven <geert@linux-m68k.org>

Changes since v3:
- Fixed compilation on SPARC & Xtensa

Changes since v2:
- Removed ifdef CONFIG_OF_OVERLAY (since for now it's required)
- Created a documentation entry
- Slight rewording in Kconfig

Changes since v1:
- of_resolve() -> of_resolve_phandles().

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
[geert: Use %zu to format size_t]
[geert: Rebase to v4.15-rc1]
[geert: Make cfs_overlay_item_dtbo_{read,write}() and
	of_cfs_overlay_group static]
[geert: Let OF_CONFIGFS select OF_FLATTREE to fix sparc all*config]
[geert: Spelling/grammar s/rationalle of/rationale for/]
[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>
Signed-off-by: Meng Li <Meng.Li@windriver.com>
Signed-off-by: Yanfei Xu <yanfei.xu@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>