e1000e: add support for LTR on I217/I218

Set the Latency Tolerance Reporting (LTR) values for the "PCIe-like"
GbE MAC in the Lynx Point PCH based on Rx buffer size and link speed
when link is up (which must not exceed the maximum latency supported
by the platform), otherwise specify there is no LTR requirement.
Unlike true-PCIe devices which set the LTR maximum snoop/no-snoop
latencies in the LTR Extended Capability Structure in the PCIe Extended
Capability register set, on this device LTR is set by writing the
equivalent snoop/no-snoop latencies in the LTRV register in the MAC and
set the SEND bit to send an Intel On-chip System Fabric sideband (IOSF-SB)
message to the PMC.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>