clocksource/drivers/arc_timer: Utilize generic sched_clock

clocksource/drivers/arc_timer: Utilize generic sched_clock

commit bf287607c80f24387fedb431a346dc67f25be12c upstream.

It turned out we used to use default implementation of sched_clock()
from kernel/sched/clock.c which was as precise as 1/HZ, i.e.
by default we had 10 msec granularity of time measurement.

Now given ARC built-in timers are clocked with the same frequency as
CPU cores we may get much higher precision of time tracking.

Thus we switch to generic sched_clock which really reads ARC hardware
counters.

This is especially helpful for measuring short events.
That's what we used to have:
------------------------------>8------------------------
$ perf stat /bin/sh -c /root/lmbench-master/bin/arc/hello > /dev/null

 Performance counter stats for '/bin/sh -c /root/lmbench-master/bin/arc/hello':

         10.000000      task-clock (msec)         #    2.832 CPUs utilized
                 1      context-switches          #    0.100 K/sec
                 1      cpu-migrations            #    0.100 K/sec
                63      page-faults               #    0.006 M/sec
           3049480      cycles                    #    0.305 GHz
           1091259      instructions              #    0.36  insn per cycle
            256828      branches                  #   25.683 M/sec
             27026      branch-misses             #   10.52% of all branches

       0.003530687 seconds time elapsed

       0.000000000 seconds user
       0.010000000 seconds sys
------------------------------>8------------------------

And now we'll see:
------------------------------>8------------------------
$ perf stat /bin/sh -c /root/lmbench-master/bin/arc/hello > /dev/null

 Performance counter stats for '/bin/sh -c /root/lmbench-master/bin/arc/hello':

          3.004322      task-clock (msec)         #    0.865 CPUs utilized
                 1      context-switches          #    0.333 K/sec
                 1      cpu-migrations            #    0.333 K/sec
                63      page-faults               #    0.021 M/sec
           2986734      cycles                    #    0.994 GHz
           1087466      instructions              #    0.36  insn per cycle
            255209      branches                  #   84.947 M/sec
             26002      branch-misses             #   10.19% of all branches

       0.003474829 seconds time elapsed

       0.003519000 seconds user
       0.000000000 seconds sys
------------------------------>8------------------------

Note how much more meaningful is the second output - time spent for
execution pretty much matches number of cycles spent (we're runnign
@ 1GHz here).

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Acked-by: Vineet Gupta <vgupta@synopsys.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>