net: stmmac: Add callbacks for DWC xpcs Energy Efficient Ethernet

Link xpcs callback functions for MAC to configure the xpcs EEE feature.

The clk_eee frequency is used to calculate the MULT_FACT_100NS. This is
to adjust the clock tic closer to 100ns.

Signed-off-by: Michael Sit Wei Hong <michael.wei.hong.sit@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>