Add HW RSS support to VNIC driver

Based on v1.0 driver provided by Cavium under BSD license.
Support in-hardware RSS to distribute IP, UDP and TCP traffic
among available RX Queues and hence multiple CPUs.

Reviewed by: wma
Obtained from: Semihalf
Sponsored by: Cavium
Differential Revision: https://reviews.freebsd.org/D6230