xtensa: replace variant/core.h with asm/core.h

Introduce the header arch/xtensa/include/asm/core.h that provides
definitions for XCHAL macros missing in older xtensa releases. Use this
header instead of variant/core.h

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
diff --git a/arch/xtensa/include/asm/vectors.h b/arch/xtensa/include/asm/vectors.h
index 7111280..79fe300 100644
--- a/arch/xtensa/include/asm/vectors.h
+++ b/arch/xtensa/include/asm/vectors.h
@@ -18,7 +18,7 @@
 #ifndef _XTENSA_VECTORS_H
 #define _XTENSA_VECTORS_H
 
-#include <variant/core.h>
+#include <asm/core.h>
 #include <asm/kmem_layout.h>
 
 #if XCHAL_HAVE_PTP_MMU