powerpc: Remove experimental status of kdump on 64-bit powerpc

This removes the experimental status of kdump on PPC64.  kdump is on
PPC64 now since more than one year and it has proven to be stable.

Signed-off-by: Bernhard Walle <bwalle@suse.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>