[POWERPC] Change ppc64_defconfig to use AUTOFS_V4 not V3

Defconfig ppc64 kernels running under late-model distros
may hang in the automounter rc.d script, which seems to be
expecting autofs version 4. This patch uses the newer autofs.

Signed-off-by: Linas Vepstas <linas@austin.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>