driver core: move to_platform_driver to platform_device.h

In converting the last remaining of_platform_driver (ibmebus) to a regular
platform driver, to_platform_driver is needed to replace
to_of_platform_driver.

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>