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>