OF: DT-Overlay configfs interface

OF: DT-Overlay configfs interface

This is a port of Pantelis Antoniou's v3 port that makes use of the
new upstreamed configfs support for binary attributes.

Original commit message:

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 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().

Originally-signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Signed-off-by: Phil Elwell <phil@raspberrypi.org>
DT configfs: Fix build errors on other platforms

Signed-off-by: Phil Elwell <phil@raspberrypi.org>
DT configfs: fix build error

There is an error when compiling rpi-4.6.y branch:
  CC      drivers/of/configfs.o
drivers/of/configfs.c:291:21: error: initialization from incompatible pointer type [-Werror=incompatible-pointer-types]
   .default_groups = of_cfs_def_groups,
                     ^
drivers/of/configfs.c:291:21: note: (near initialization for 'of_cfs_subsys.su_group.default_groups.next')

The .default_groups is linked list since commit
1ae1602de028acaa42a0f6ff18d19756f8e825c6.
This commit uses configfs_add_default_group to fix this problem.

Signed-off-by: Slawomir Stepien <sst@poczta.fm>
configfs: New of_overlay API