Just like the rest of the subsystems, let's add the required no-op
functions to implement stubs when CONFIG_OF=n.

This prevents MTD drivers from having ugly ifdefs in their code,
and instead hide the ifdef monster in the header closet (far away
from people's sight).
Signed-off-by: NEzequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: NBrian Norris <computersforpeace@gmail.com>