C6X: replace tick_nohz_stop/restart_sched_tick calls

The following commits replaced the tick_nohz_{stop,restart}_sched_tick
API with separate tick and rcu calls:

  280f0677
  2bbb6817
  1268fbc7

This patch replaces the C6X use of the old API with the newer interfaces.

Signed-off-by: Mark Salter <msalter@redhat.com>