MIPS: jz4740: Implement read_sched_clock

Use jz4740 timer 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/9485/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>