MIPS: Enable ptrace hw watchpoints on MIPS R6

MIPS: Enable ptrace hw watchpoints on MIPS R6

HARDWARE_WATCHPOINTS isn't being enabled for CPU_MIPSR6, even though it
has an identical hardware watchpoint interface to CPU_MIPSR2, which
prevents ptrace watchpoints from being loaded when executing a ptraced
process even though the watchpoints are described in /proc/cpuinfo.

Enable HARDWARE_WATCHPOINTS for CPU_MIPSR6 too.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/12727/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>