Add DWC xpcs EEE support callbacks.The callback function is used to
set EEE registers on xpcs.

xpcs transparent mode is enabled to allow PHY to detect MAC EEE status.
Signed-off-by: Michael Sit Wei Hong <michael.wei.hong.sit@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>