[POWERPC] Create modalias file in sysfs for of_platform bus

Create /sys/bus/of_platform/devices/*/modalias file to allow autoloading
of modules.  Modalias files are already present for many other bus types.
This adds also a newline to the devspec files.

Also create a devspec file for mac-io devices.  They were created as a
side effect.  Use correct buffer size for mac-io modalias buffer.

Tested on iBook1 and Efika.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>