cxgbe.4: Update for recent HARDWARE

Adjust document description, description section, and hardware section
to include Chelsio T7. Since this driver now supports many generations,
order reverse chronologically. Compare language to ice.4 and bnxt.4,
the other terabit ethernet drivers.

This list of HARDWARE is obtained from the Chelsio prouduct selector.

MFC after: 3 days (T7 support shipped with 15.0)
Discussed with: jhb, np
Differential Revision: https://reviews.freebsd.org/D54276