BACKPORT: of: make CONFIG_OF user selectable

With the addition of overlays, it is now plausible to use DT on any arch
and without an arch using it at boot time. It is also desirable to
expand the compile coverage of the DT code. Make CONFIG_OF user
selectable by converting the menu to menuconfig.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Acked-by: Grant Likely <grant.likely@lianro.org>
(cherry picked from commit 0166dc11be911213e0b1b764488c671be4c48cf3)
[astrachan: dropped OF_OVERLAY hunk; not present on 3.18]
Bug: 70214720
Change-Id: Iabaa07bb52c0bbc4fd95949d4a5b20ac8bc74193
Signed-off-by: Alistair Strachan <astrachan@google.com>