sfxge(4): add Medford2 head-of-line blocking stats

These stats are availble on Medford2 DPDK firmware variant
which support equal stride super-buffer Rx mode. RXDP_HLB_IDLE
capability bit is set when the stats are available.

Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18243