net: pcs: add C37 SGMII AN support for intel mGbE controller

XPCS IP supports C37 SGMII AN process and it is used in intel multi-GbE
controller as MAC-side SGMII.

Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>