MIPS: oprofile: Enable backtrace on timer-based profiling

Allow unsupported CPU types to use backtrace with timer-based profiling.
Some CPUs (notably OCTEON) lack architecture-specific oprofile driver. In
such case oprofile can fallback to timer-based mode, and arch code can
still provide the backtrace functionality. So just set up the backtrace
hook always.

Signed-off-by: Aaro Koskinen <aaro.koskinen@nsn.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8108/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>