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>

arch/xtensa/kernel/head.S

@@ -124,7 +124,7 @@ ENTRY(_startup)
 
 	/* Disable coprocessors. */
 
-#if XCHAL_CP_NUM > 0
+#if XCHAL_HAVE_CP
 	wsr	a0, cpenable
 #endif