dt: reform for_each_property to for_each_property_of_node

Make this macro easier to use(do not need to pass properties, a node is
enough), also change to a more sensible name as for_each_child_of_node.

Signed-off-by: Dong Aisheng <dong.aisheng@linaro.org>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>