i2c-designware: add OF binding support

Add of_match_table and DT style i2c registration to designware i2c
driver.

Refactored for pci/plat split by Dirk Brandewie.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Dirk Brandewie <dirk.brandewie@gmail.com>