amd-xgbe: Add support for a KR redriver

This patch provides support for the presence of a KR redriver chip in
between the device PCS and an external PHY.  When a redriver chip is
present the device must perform clause 73 auto-negotiation in order to
set the redriver chip for the downstream connection.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>