Add PPC_QEMU_E500 to corenet_base.config, which is then used to generate
corenet64_smp_defconfig and corenet32_smp_defconfig.

That then allows both those configs to build kernels that boot in qemu
using the ppce500 machine type and respectively -cpu e5500 or -cpu
e500mc.

The code that is added by PPC_QEMU_E500 just defines another machine
with a probe function that recognises qemu, so there should be no change
when booting on actual hardware supported by CORENET_GENERIC.

The increase in vmlinux size is less than 1KB.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230411102838.512859-1-mpe@ellerman.id.au