[POWERPC] Move serial_dev_init to device_initcall()

With the I/O space rewrite by BenH, the legacy_serial serial_dev_init()
initcall is now called before I/O space is setup, but it's dependent on
it being available.

Since there's no way to make dependencies between initcalls, we'll just
have to move it to device_initcall(). Yes, it's suboptimal but I'm not
aware of any better solution at this time, and it fixes a regression
from 2.6.22.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>