powerpc/85xx: Update corenet64_smp_defconfig for T4240

* Add support for up to 24 cores on T4240 (includes threads)
* Enable AltiVec support (on T4240)
* Add T4240QDS board into build
* Other changes are due to general kernel update of defconfig

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>