bnxt_re: User library support for RoCE driver

This patch introduces userspace library support for the bnxt_re RoCE driver.
The library can be linked with RDMA applications such as perftest and rping.
The RoCE traffic has been tested with the rping and perftest utility.

Reviewed by: imp, kib, sumit.saxena@broadcom.com
Approved by: imp
Differential revision: https://reviews.freebsd.org/D45729