dt: add empty of_property_read_u32[_array] for non-dt

The patch adds empty functions of_property_read_u32 and
of_property_read_u32_array for non-dt build, so that drivers
migrating to dt can save some '#ifdef CONFIG_OF'.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
[grant.likely: Moved things around so only one new static inline is needed]
[grant.likely: Added _string variant]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>