[POWERPC] Rename get_property to of_get_property: the last one

This also fixes a bug where a property value was being modified
in place.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>