event/octeontx: start and stop timer device

event/octeontx: start and stop timer device

When application requests to start the timer adapter through
`rte_event_timer_adapter_start`, Octeontx TIMvf ring does the
following:
- Uses mbox to communicate TIMpf driver about,
  * SCLK frequency used to convert ns<->cycles.
  * program the ring control parameters and start the ring.
  * get the exact cycle at which the TIMvf ring has started which can be
  used to estimate the bucket position.

On `rte_event_timer_adapter_stop` i.e stop, Octeontx TIMvf ring does the
following:
- Use mbox to communicate TIMpf driver about,
  * reset the ring control parameters and stop the ring.

Signed-off-by: Pavan Nikhilesh <pbhagavatula@caviumnetworks.com>