xtensa: clear all DBREAKC registers on start

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

Cc: stable@vger.kernel.org
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>