dt/serial: Eliminate users of of_platform_{,un}register_driver

Get rid of users of of_platform_driver in drivers/serial.  The
of_platform_{,un}register_driver functions are going away, so the
users need to be converted to using the platform_bus_type directly.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>