powerpc: Don't build powernv for other platform defconfigs

Because powernv arrived after these other platforms, the defconfigs
didn't have PPC_POWERNV disabled, and being default y it gets turned on.

If we're going to bother having defconfigs for the specific platforms
then they should only build the code required for those platforms.

The grab bag of everything config is ppc64_defconfig.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>