ethdev: enhance burst mode information API

ethdev: enhance burst mode information API

Change the type of burst mode information from bit field to free string
data, so that each PMD can describe the Rx/Tx busrt functions flexibly.

Fixes: eb5902504a13 ("ethdev: add API for getting burst mode information")
Fixes: 6b6609f68ccd ("net/i40e: support Rx/Tx burst mode info")
Fixes: e9a10e6c2102 ("net/ice: support Rx/Tx burst mode info")
Fixes: 7fe108edcf53 ("app/testpmd: show Rx/Tx burst mode description")

Signed-off-by: Haiyue Wang <haiyue.wang@intel.com>
Acked-by: Ray Kinsella <ray.kinsella@intel.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>