ethdev: introduce available Rx descriptors threshold

ethdev: introduce available Rx descriptors threshold

A new event RTE_ETH_EVENT_RX_AVAIL_THRESH should be generated by HW
when number of available descriptors in Rx queue goes below the
threshold.

The threshold is defined as a percentage of an Rx queue size with valid
values from 0 to 99 (inclusive). Zero (default) value disables it.

There is no capability reporting for the feature. Application should
simply try to set required threshold value and handle result.

Add testpmd commands to control the threshold:
  set port <port_id> rxq <rxq_id> avail_thresh <avail_thresh_num>

Signed-off-by: Spike Du <spiked@nvidia.com>
Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Thomas Monjalon <thomas@monjalon.net>