xtensa: initialize CPENABLE SR when core has one

XCHAL_CP_NUM is defined in variant/tie.h and it is not included by
head.S, leaving CPENABLE register uninitialised. XCHAL_HAVE_CP is
defined in variant/core.h to 1 when core has CPENABLE SR.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>