ipmi: convert OF driver to platform driver

of_bus is deprecated in favor of the plain platform bus. This patch
merges the ipmi OF driver with the existing platform driver.

CONFIG_PPC_OF occurrances are removed or replaced with CONFIG_OF.

Compile tested with and without CONFIG_OF. Tested OF probe and
default probe cases.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>