net/i40e/base: add function to read LPI statistics

net/i40e/base: add function to read LPI statistics

There was no time duration statistics for LPI (Low Power Idle)
in EEE (Energy-Efficient Ethernet).
Added new procedure to get duration values from FW.
Otherwise there is no available procedure to get tx_lpi_duration
and rx_lpi_duration values.

Signed-off-by: Jaroslaw Gawin <jaroslawx.gawin@intel.com>
Signed-off-by: Xiaolong Ye <xiaolong.ye@intel.com>
Acked-by: Qi Zhang <qi.z.zhang@intel.com>
Acked-by: Beilei Xing <beilei.xing@intel.com>