dt: eliminate of_platform_driver shim code

Commit eca393016, "of: Merge of_platform_bus_type with
platform_bus_type" added a shim to allow of_platform_drivers to get
registers onto the platform bus so that there was time to migrate the
existing drivers to the platform_bus_type.

This patch removes the shim since there are no more users of the old
interface.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>