xtensa: introduce prep_c for xtensa

xtensa is the only architecutre doing thing differently and introduces
inconsistency in the init process and dependencies as we attemp to
cleanup init levels and remove misused of SYS_INIT.

Introduce prep_c for this architecture and align with other
architectures.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
3 files changed