powerpc: Cleanup NLS config options on pseries, ppc64 and ppc64e defconfig

Set CONFIG_NLS_DEFAULT to utf8. The distros do this (eg ppc64 FC17
and RHEL6) as well as the x86 defconfigs. Userspace these days is
most likely to expect utf8 anyway.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>