MIPS: csrc-ioasic: Implement read_sched_clock

Use DEC I/O ASIC's free-running counter for sched_clock source. This
implementation will give high resolution cputime accounting.

Acked-by: Maciej W. Rozycki <macro@linux-mips.org>
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/9482/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

diff --cc arch/mips/kernel/csrc-ioasic.c
Simple merge