MIPS: Add sched_clock support

MIPS: Add sched_clock support

This will provide sched_clock interface to implement individual
read_sched_clock(). Not for CAVIUM_OCTEON_SOC as it defines its own
sched_clock() directly (not using the sched_clock_register interface).

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/9477/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>