of: make unittest select OF_EARLY_FLATTREE instead of depend on it

The DT unittest currently requires an arch (typically) to select
OF_EARLY_FLATTREE. Remove this dependency by selecting it directly so that
the unittest can be enabled easily on any architecture. With this and the
prior commit, we can easily enable and run unittests starting with x86
defconfig rather than hunting for the combination of config options to
enable OF on x86.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Acked-by: Grant Likely <grant.likely@lianro.org>