ARM: pxa3xx: skip default device initialization when booting via DT

When booting via DT, the default PXA devices must not have been probed
before, otherwise the augmented information from the device tree is
ignored.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>