arch: add comments to empty default case, add default LOG_ERR

According to the Zephyr Coding Guideline all switch statements
shall be well-formed.
Add a comment to the empty default case.
Add a LOG_ERR to the default case.

Found as a coding guideline violation (MISRA R16.1) by static
coding scanning tool.

Signed-off-by: Maksim Masalski <maksim.masalski@intel.com>