MIPS: cevt-txx9: Implement read_sched_clock

Use txx9 up-counter for sched_clock source. This implementation will give
high resolution cputime accounting.

Signed-off-by: Deng-Cheng Zhu <dengcheng.zhu@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: macro@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9484/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>