of: Empty node & property flag accessors when !OF

Introduce empty node and property flag accessors when CONFIG_OF is not
defined.  This allows us to use them without ifdef'ing them in places
where it makes sense to do so.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Grant Likely <grant.likely@linaro.org>