soc/intel_adsp: Put a guard around CAVS_VERSION

This is some SOF code that got imported into Zephyr.  But it's not a
Zephyr API and Zephyr doesn't use it.  Unfortunately, the Zephyr
definition is now the one actually used at runtime in SOF, so we can't
remove it.

Put a guard around the definition so nothing else uses it until we get
it moved back home.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>