boards: xtensa: rename qemu_xtensa_dc233c to qemu_xtensa_mmu

This renames the board from qemu_xtensa_dc233c to
qemu_xtensa_mmu to better signal that it is for testing with
MMU on QEMU Xtensa. Also turn on testing by default to make
sure future changes will not break Xtensa MMU support.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>