[POWERPC] change get_property to return void *

Change the get_property() function to return a void *. This allows us
to later remove the cast done in the majority of callers.

Built for pseries, iseries, pmac32, cell, cbesim, g5, systemsim, maple,
and mpc* defconfigs

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>