of: remove of_platform_driver

The last user of of_platform_driver is converted to a regular
platform_driver, so of_platform_driver can be removed now.

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>