bxe: enable usage with NetXtreme II BCM57840 2x20GbE chip

Current bxe probe function won't attach to devices with the NetXtreme II
BCM57840 2x20GbE chip, enable it by adding it's chip ID to the list of
supported chips.

Tested on: HP ProLiant WS460c Gen9
Reviewed by: gnn
MFC after: 1 week
Sponsored by: Citrix Systems R&D
Differential Revision: https://reviews.freebsd.org/D9609