[POWERPC] Export of_device_get_modalias

Apparently other parts of the kernel need to know the
modalias internally (like the sysfs code in macintosh driver).

To avoid consistency issues, we export this code and use it
everywhere it's needed rather than repeat it ...

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>