bnxt_re/libbnxtre: Add RoCE support for BCM576xx controllers

Added RoCE support for BCM576xx controllers with below changes,

1. Update the BAR offsets for handling BCM576xx controllers.

Use the values populated by the L2 driver for getting the
Doorbell offsets.

2. Use msn index instead of tail to pull psn table entry.

3. Temporarily disable dbr pacing feature untill it is

fully implemented.

4. Add support for 400G speed.

Reviewed by: ssaxena
Differential Revision: https://reviews.freebsd.org/D54521
MFC after: 3 days