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>

arch/xtensa/kernel/hw_breakpoint.c

@@ -12,7 +12,7 @@
 #include <linux/log2.h>
 #include <linux/percpu.h>
 #include <linux/perf_event.h>
-#include <variant/core.h>
+#include <asm/core.h>
 
 /* Breakpoint currently in use for each IBREAKA. */
 static DEFINE_PER_CPU(struct perf_event *, bp_on_reg[XCHAL_NUM_IBREAK]);