dt: add empty of_get_property for non-dt

The patch adds empty function of_get_property for non-dt build, so that
drivers migrating to dt can save some '#ifdef CONFIG_OF'.

This also fixes the current Tegra compile problem in linux-next.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>