fbnic: Add SW shim for MDIO interface to PMD and PCS

fbnic: Add SW shim for MDIO interface to PMD and PCS

In order for us to support a PCS device we need to add an MDIO bus to allow
the drivers to have access to the registers for the device.  This change
adds such an interface.

The interface will consist of 2 PHY addrs, the first one consisting of a
PMD and PCS, and the second just being a PCS. There is a need for 2 PHYs
addrs due to the fact that in order to support the 50GBase-CR2 mode we will
need to access and configure the PCS vendor registers and RSFEC registers
from the second lane identical to the first.

Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Link: https://patch.msgid.link/176374324532.959489.15389723111560978054.stgit@ahduyck-xeon-server.home.arpa
Signed-off-by: Paolo Abeni <pabeni@redhat.com>