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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>