ibmebus: convert of_platform_driver to platform_driver

ibmebus is the last remaining user of of_platform_driver and the
conversion to a regular platform driver is trivial.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Grant Likely <grant.likely@linaro.org>