powerpc: Rename e55xx_smp_defconfig to corenet64_smp_defconfig

Rather than trying to use the core name we use corenet to distinquish
the platform/core combo.  corenet64 will be a 64-bit kernel build and
we'll add a new defconfig for corenet32 for a 32-bit platforms.
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>