stmmac: intel: add cross time-stamping freq difference adjustment

Cross time-stamping mechanism used in certain instance of Intel mGbE
may run at different clock frequency in comparison to the clock
frequency used by processor, so we introduce cross T/S frequency
adjustment to ensure TSC calculation is correct when processor got the
cross time-stamps.

Signed-off-by: Wong Vee Khee <vee.khee.wong@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>