boards: qemu_xtensa: use dc233c core

This changes qemu_xtensa to use dc233c core instead of
sample_controller. The sample_controller uses a very
basic configuration which lacks features usually needed
in real world applications. Instead, use the dc233c core
as the base for qemu_xtensa so we can use QEMU to cover
more of our code path.

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