MIPS: csrc-bcm1480: Implement read_sched_clock

Use the ZBbus cycle 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/9480/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

diff --cc arch/mips/kernel/csrc-bcm1480.c
Simple merge