eth: fbnic: Add link detection

eth: fbnic: Add link detection

Add basic support for detecting the link and reporting it at the netdev
layer. For now we will just use the values reporeted by the firmware as the
link configuration and assume that is the current configuration of the MAC
and PCS.

With this we start the stubbing out of the phylink interface that will be
used to provide the configuration interface for ethtool in a future patch
set.

The phylink interface isn't an exact fit. As such we are currently working
around several issues in this patch set that we plan to address in the
future such as:
1. Support for FEC
2. Support for multiple lanes to handle 50GbaseR2 vs 50GbaseR1
3. Support for BMC

CC: Russell King <linux@armlinux.org.uk>
CC: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Link: https://patch.msgid.link/172079939835.1778861.5964790909718481811.stgit@ahduyck-xeon-server.home.arpa
Signed-off-by: Jakub Kicinski <kuba@kernel.org>