target/xtensa: allow different default CPU for MMU/noMMU

Define default core for noMMU configurations and use that core as
machine default with noMMU XTFPGA machines.
This is done to avoid offering non-working configuration (MMU core on a
noMMU machine) as a default.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
2 files changed