ixl(4): Add support for X710-T*L devices

Add support for new devices which are capable of 2.5 and 5G speeds, as well as
Energy Efficient Ethernet (EEE):

• introduce new device ids
• add ability to select 2.5 and 5G speeds on devices which support it
• add sysctls to enable EEE and read related statistics

Submitted by: Krzysztof Galazka <krzysztof.galazka@intel.com>
Reviewed by: #IntelNetworking
MFC after: 3 days
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D25549