xtensa: clear all DBREAKC registers on start

commit 7de7ac78 upstream.

There are XCHAL_NUM_DBREAK registers, clear them all.
This also fixes cryptic assembler error message with binutils 2.25 when
XCHAL_NUM_DBREAK is 0:

  as: out of memory allocating 18446744073709551575 bytes after a total
  of 495616 bytes
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

arch/xtensa/kernel/head.S

@@ -123,7 +123,7 @@ ENTRY(_startup)
 	wsr	a0, icountlevel
 
 	.set	_index, 0
-	.rept	XCHAL_NUM_DBREAK - 1
+	.rept	XCHAL_NUM_DBREAK
 	wsr	a0, SREG_DBREAKC + _index
 	.set	_index, _index + 1
 	.endr