net: pcs: Introducing support for DWC xpcs Energy Efficient Ethernet

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>