Added support for displaying HW port stats using sysctl.

This provides port stats (updated once per second) in
dev.bnxt.X.port_stats for PFs. VFs do not have access to the port stats.

Submitted by: Bhargava Chenna Marreddy <bhargava.marreddy@broadcom.com>
Reviewed by: shurd, sbruno
Approved by: sbruno (mentor)
Sponsored by: Broadcom Limited
Differential Revision: https://reviews.freebsd.org/D11914