powerpc: Add a defconfig for 'corenet' 32-bit platforms

The e500mc and e5500 based cores are only available on corenet based
SoCs.  We use this name for the P204x, P3040, P4040, P4080, P50x0 SoCs
and any future processors in these families.

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