csky: Add context tracking support

This patch support context tracking with no hz full.

Here is the test result with dynticks-testing (see tick_stop):

 cat /sys/kernel/debug/tracing/per_cpu/cpu0/trace
 tracer: nop

 entries-in-buffer/entries-written: 356/356   #P:1

                              _-----=> irqs-off
                             / _----=> need-resched
                            | / _---=> hardirq/softirq
                            || / _--=> preempt-depth
                            ||| /     delay
           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
              | |       |   ||||       |         |
...
          sleep-192   [000] d.h.   167.088270: hrtimer_expire_entry: hrtimer=(ptrval) function=tick_sched_timer now=166436355700
          sleep-192   [000] d.h.   167.092279: hrtimer_expire_entry: hrtimer=(ptrval) function=tick_sched_timer now=166440365700
         <idle>-0     [000] d.h2   167.096492: hrtimer_expire_entry: hrtimer=(ptrval) function=tick_sched_timer now=166444578400
         <idle>-0     [000] d..1   167.097876: tick_stop: success=1 dependency=NONE
                                               ^^^^^^^^^
         <idle>-0     [000] d.h1   168.818206: hrtimer_expire_entry: hrtimer=(ptrval) function=tick_sched_timer now=168166280900
   kworker/u2:0-7     [000] ....   168.821760: workqueue_execute_start: work struct (ptrval): function wb_workfn
   kworker/u2:0-7     [000] d.h1   168.824464: hrtimer_expire_entry: hrtimer=(ptrval) function=tick_sched_timer now=168172547100
    kworker/0:1-18    [000] ....   168.825053: workqueue_execute_start: work struct (ptrval): function vmstat_update
    kworker/0:1-18    [000] ....   168.825238: workqueue_execute_start: work struct (ptrval): function vmstat_shepherd
    kworker/0:1-18    [000] ....   168.825516: workqueue_execute_start: work struct (ptrval): function neigh_periodic_work
         <idle>-0     [000] d..1   168.826121: tick_stop: success=1 dependency=NONE
   kworker/u2:0-7     [000] ....   169.377327: workqueue_execute_start: work struct (ptrval): function flush_to_ldisc
         <idle>-0     [000] d..1   169.379832: tick_stop: success=1 dependency=NONE
   kworker/u2:0-7     [000] ....   169.607935: workqueue_execute_start: work struct (ptrval): function flush_to_ldisc
   kworker/u2:0-7     [000] d.h1   169.608148: hrtimer_expire_entry: hrtimer=(ptrval) function=tick_sched_timer now=168956235500
         <idle>-0     [000] d..1   169.608654: tick_stop: success=1 dependency=NONE

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Cc: Greentime Hu <greentime.hu@sifive.com>
Cc: Arnd Bergmann <arnd@arndb.de>