net: eth: fbnic: Add pma read and write access

net: eth: fbnic: Add pma read and write access

Document the MDIO interface topology with an ASCII diagram
showing the MAC, PCS (MMD 3), FEC, Separated PMA (MMD 8), and PMD
(MMD 1) blocks and their interconnects. The diagram illustrates how
4 lanes connect the MAC through PCS, FEC, and PMA, then narrow to
2 lanes at the PMD.

The c45 read and write routines are enhanced to support
read and write of the separated PMA for the fbnic.

Co-developed-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: Mike Marciniszyn (Meta) <mike.marciniszyn@gmail.com>
Link: https://patch.msgid.link/20260430150802.3521-4-mike.marciniszyn@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>