amd-xgbe: Add I2C support for sideband communication

Add support to initialize and use the I2C controller within the hardware
in order to perform sideband communication, e.g. determine the SFP media
type that is installed.

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