powerpc/configs: Enable THP and 64K for ppc64(le)_defconfig

Enable 64K page size and THP. I use ppc64le_defconfig when I need
a single config across guest and host, but having 4K page size
as default is not what I expect. I could move these over to
server.config and merge if ppc64_defconfig is meant for systems
that use 4k pages by default.

Signed-off-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>