mlxbf_gige: add MDIO support for BlueField-3

This patch adds initial MDIO support for the BlueField-3
SoC. Separate header files for the BlueField-2 and the
BlueField-3 SoCs have been created. These header files
hold the SoC-specific MDIO macros since the register
offsets and bit fields have changed. Also, in BlueField-3
there is a separate register for writing and reading the
MDIO data. Finally, instead of having "if" statements
everywhere to differentiate between SoC-specific logic,
a mlxbf_gige_mdio_gw_t struct was created for this purpose.

Signed-off-by: David Thompson <davthompson@nvidia.com>
Signed-off-by: Asmaa Mnebhi <asmaa@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>