amd-xgbe-phy: Allow certain PHY settings to be set by UEFI

Certain PHY settings need to be configurable by UEFI depending on the
platform being used.  Add new device tree / ACPI properties that, if
present, will override the pre-determined values currently used.

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