mlxbf-gige: support fixed phy for Bobcat

[Asmaa Mnebhi]

SRU Justification:

[Impact]

Bobcat is a new board which doesn't have an external PHY connected to the OOB. There are no MDIO busses involved and no PHY involved. The OOB is directly connected to the switch (MAC to MAC). SO we need to use the linux fixed phy to be able to emulate the mdio behavior and load the ethernet driver.

[Fix]

* Register the fixed-phy register in the case of the bobcat.

[Test Case]

* Important: For testing on the bobcat board, make sure the corresponding UEFI image is also loaded, otherwise, the oob driver will fail to load. For other board, it is backward compatible.
* Check if the mlxbf-gige is loaded on bobcat
* Check that it is using the fixed-phy via dmesg | grep PHY
* check that the oob interface is pingable
* check that files can be copied over via oob interface
* rmmod/modprobe
* ifconfig up/down
* reboot test

[Regression Potential]

* Redo all the OOB testing on other boards (moonraker) and make sure the bobcat changes dont trigger any regressions.

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This bug is awaiting verification that the linux-bluefield/5.15.0-1039.41 kernel in -proposed solves the problem. Please test the kernel and update this bug with the results. If the problem is solved, change the tag 'verification-needed-jammy-linux-bluefield' to 'verification-done-jammy-linux-bluefield'. If the problem still exists, change the tag 'verification-needed-jammy-linux-bluefield' to 'verification-failed-jammy-linux-bluefield'.

If verification is not done by 5 working days from today, this fix will be dropped from the source code, and this bug will be closed.

See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you!