spi/pxa2xx: register driver properly

use platform_driver_register instead of platform_driver_probe. The
latter only checks available devices at the time of calling. So if a
device gets inserter at a later point in time then the driver will never
play with it.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Dirk Brandewie <dirk.brandewie@gmail.com>