dt/platform: minor cleanup

* Correct description of of_platform_bus_create to match implementation
* Remove a level of indentation in of_dev_lookup

Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>